added Settings usages examples

doc/docs/beacon_settings.md

 # Beacon settings
 
+## Settings
+
 *Beacon settings* are helper classes to store data in the Redis database at
 runtime from Python code, using BLISS.
 
@@ -8,67 +10,184 @@ Use cases for settings:
 * on top of static configuration parameters
     - motor velocity, acceleration, limits
     - auto-gain on a Keithley
-* to keep (or share) information across executions (or across processes)
+
+* to persistency keep (or share) information across executions (or
+  across processes)
     - selected counter for plot
     - enabled loggers, log level
     - scan saving path, file template...
 
-## Settings classes
+
+!!! note
+    To access to the redis database:
+    
+    `redis-cli -s /tmp/redis.sock -n 0`
+
+
 
 BLISS provides different classes for different kinds of settings.
 
-### SimpleSetting
+## SimpleSetting
 
-The `SimpleSetting` class is used to save a *scalar value*:
+The `SimpleSetting` class is used to store a *scalar value*:
 
 * int
 * float
 * string
 * bool
 
+The typical usages are:
+
+* to create the setting with a default value:
+
+       `sss = SimpleSetting(<key>, default_value=<value>)`
+
+* to set a value to store: `sss.set()`
+* to read the stored value: `sss.get()`)
+* to reset the value to the default one: `sss.clear()`
+
 Example:
 
-```py
+```python
 from bliss.config import settings
 magicNumber = 63825363
-iii = settings.SimpleSetting('myIntkey', default_value=magicNumber)
-assert iii.get() == magicNumber
-iii.set(42)
-assert iii.get() == 42
-assert isinstance(iii.get(), int)
+sss = settings.SimpleSetting('myIntkey', default_value=magicNumber)
+print(sss)
+# <SimpleSetting name=myIntkey value=None>
+print(sss.get())
+# 63825363    # this is the default value.
+sss.set(42)
+
+print(sss)
+# <SimpleSetting name=myIntkey value=b'42'>
+
+print(sss.get())
+# 42
+
+sss.clear()
+print(sss.get())
+# 63825363
+
+sss.set(3.14)
+```
+
+`redis-cli` can be used to inspect the redis database:
+```
+% redis-cli -s /tmp/redis.sock -n 0
+redis /tmp/redis.sock> keys my*
+1) "myHkey"
+2) "myIntkey"
+
+redis /tmp/redis.sock> get myIntkey
+"3.14"
+```
+
+After a restart of the session (or from another session):
+
+```python
+from bliss.config import settings
+sss = settings.SimpleSetting('myIntkey')
+print(sss.get())
+# 3.14
 ```
 
-The `default_value` is returned by the `SimpleSetting` object until it has been
-set. Once the object has been set, the value is persisted in redis.
+The `default_value` is returned by the `SimpleSetting` object until it
+has been set or after a `clear()`. Once the object has been set, the
+value is persisted in redis.
 
-### BaseHashSetting and HashSetting
+Note that `default_value` can be `None`, but a `SimpleSetting` cannot
+be set to `None`.
 
-The `BaseHashSetting` class is used to represent a dictionary of scalar values,
-`HashSetting` simply adds a kwarg `default_values` that is a dictionary containing
-values taken as a fallback.
+After a `.clear()`, the key is removed from Redis database:
 
-```py
-myDict = {"C1":"riri", "C2":"fifi"}
-shs = settings.HashSetting('myHkey', default_values=myDict)  # note the s :)
+```python
+sss.clear()
+```
+
+```
+redis /tmp/redis.sock> keys myIntkey
+(empty list or set)
 ```
 
-### QueueSetting
+
+## BaseHashSetting and HashSetting
+
+The `BaseHashSetting` class is used to represent a *dictionary* of
+scalar values. `HashSetting` simply adds a kwarg `default_values` that
+is a dictionary containing values taken as a fallback.
+
+```python
+from bliss.config import settings
+myDict = {"key1":"value1", "key2":"value2"}
+hs = settings.HashSetting('myHkey', default_values=myDict)
+                                      # note the 's' in default_values
+# 'hs' acts now as a dictionary.
+print(hs["key1"])
+# value1
+print(hs["key2"])
+# value2
+hs["key1"]=3333
+print(hs["key1"])
+# 3333
+```
+
+Redis stores only key/value pairs that have changed:
+
+```
+% redis-cli -s /tmp/redis.sock -n 0
+redis /tmp/redis.sock> hgetall myHkey
+1) "key1"
+2) "3333"
+```
+
+After a `.clear()`, the key is removed from Redis database.
+
+A key can be added and removed:
+
+bliss session:
+```python
+hs["newKey"]=45
+```
+
+redis:
+```
+hgetall myHkey
+1) "key1"
+2) "3333"
+3) "newKey"
+4) "45"
+```
+
+bliss session:
+```python
+hs.remove("newKey")
+```
+
+redis:
+```
+redis /tmp/redis.sock> hgetall myHkey
+1) "key1"
+2) "3333"
+```
+
+
+## QueueSetting
 
 The `QueueSetting` class is used to represent a list of scalar values:
 
-```py
+```python
 >>> myList = ["a", "b", "c", "d"]
 >>> sqs = settings.QueueSetting('myQkey')
 >>> sqs.set(myList)
 ```
 
-### ParametersWardrobe
+## ParametersWardrobe
 
 `ParametersWardrobe` are more advanced objects -- it is used to group
 simple settings, and to be able to switch from one set of values for
 the parameters to another:
 
-```py
+```python
 >>> from bliss.config.settings import ParametersWardrobe
 >>> p = ParametersWardrobe("my_params")
 >>> p.add("test", 42)