Musst enhancement

Added some feature:
 - possible to configure musst channels (counters,encoder) in configuration. 
 - get the program root path from the configuration.
 - don't reload the same program twice (use the Cache feature).
 - print an error message if the program failed to load.

please merge !226 first

See merge request !227

Cache value

Define in the system a tools to cache variable with the ability to clean cache value.

See merge request !226

if a program is running.

Those input channel may now be setup in the configuration.
i.e: musst.get_channel_by_name('motorX').

Cached the last program file name with Channels.
Don't re-upload the program if it's the same name.

New Cache object base an a simple channel.
To create a cache value, the device new to have a *name* attribute

motor: acquisition jog control

Jog feature for acquisition motor master

See merge request !228

Fix for issue #44

Let scripts using the 'old' API to work with the newest `data_manager.py`.

Tested on ID29.

This old API should be deprecated but at the moment it is easier to just be
compatible since it requires changes in scripts that are in production.

I also did something if `env` is not specified: it creates a default env.


See merge request !225

URGENT: fixed typos in comm/*.py files

Some `import` statements were wrong.

I decided to change the file called `common.py` in comm/ directory to `exceptions.py` since
it contained only Exception classes. This is to avoid confusion with other files called
`common.py`. Hope you agree.


See merge request !224

added back cleanup and error_cleanup imports in bliss/__init__.py

Those 2 got removed, breaking scripts on id29.

Now they're back !

See merge request !219

Bliss config plugin

small modification to create referenced object

See merge request !222

is still running on some beamlines (MX)

axis: add property to get encoder step per unit.

helper to retrieve encoder step per unit.
This is handy when you want to resynchronise a device (like the musst)
which read axis encoder.

See merge request !223

create referenced objects of a sub configuration list.

This may cause some problem if we try to save the modify node.

fix issue-59: error in temperature code introduced by #170



See merge request !221

Fix #58: Tango Emotion: dev_status exec. axis.state() code in gevent thread

The issue #58 seems to be fixed with this patch. The test script output is:

```
OK: 1425
ON READY (Axis is READY) 0.971999999999
OK: 1338
ON READY (Axis is READY) -0.971999999999
OK: 1097
ON READY (Axis is READY) 0.971999999999
OK: 927
ON READY (Axis is READY) -0.971999999999
OK: 1135
ON READY (Axis is READY) 0.971999999999
OK: 1318
ON READY (Axis is READY) -0.971999999999
OK: 887
ON READY (Axis is READY) 0.971999999999
OK: 923
ON READY (Axis is READY) -0.971999999999
OK: 1240
ON READY (Axis is READY) 0.971999999999
OK: 1408
ON READY (Axis is READY) -0.971999999999
OK: 1342
ON READY (Axis is READY) 0.971999999999
OK: 1293
ON READY (Axis is READY) -0.971999999999
OK: 1475
ON READY (Axis is READY) 0.971999999999
OK: 1278
ON READY (Axis is READY) -0.971999999999
OK: 925
ON READY (Axis is READY) 0.971999999999
OK: 695
ON READY (Axis is READY) -0.971999999999
OK: 656
ON READY (Axis is READY) 0.971999999999
OK: 627
ON READY (Axis is READY) -0.971999999999
OK: 663
ON READY (Axis is READY) 0.971999999999
OK: 642
ON READY (Axis is READY) -0.971999999999
```

See merge request !220

aerotech: first version of A3200 over Taco server



See merge request !216

`stop_jog` method in motor controller and bug fix with jog direction if steps_per_unit < 0



See merge request !218

common/axis.py: start_jog is now called with 'right' direction, taking sign of steps_per_unit into account

of 'stop' on controller when doing jog motion - by default, 'stop_jog' == 'stop'

opiom: fixed a bug in communication initialization.



See merge request !217