MainSession is a 'proxy' to the current loaded session, or to a default,
empty one. It is useful to solve import problems, when modules depends
on each other. The main session is the top level session.

In the future there might be more things in the main module.

Do not need to bother with 'gc' anymore

counters test: use simulation diode objects / bug fix in measurement.py: take into account conversion function in case a grouped read handler exists

Issue #615
* No '.' in names
* roi counter renaming with '_'
* simple counter namespace implementation
* counter fullname contains counter name

Aliases refactoring
* removed AliasMixin class
* create_alias becomes a global function in alias module
* removed export_to_globals, remove_original options (set by default)
* always keep a strong ref to the underlying object in Alias
* removed 'hide_controller' option
* removed 'alias_or_fullname, alias_or_name properties
    - code explicitely ask for name, fullname or alias instead

Moved get_* functions from utils to session

redis-py uses 'poll'-based selector as a default ; 'poll' implementation
in gevent seems to have a problem with redis-py, not sure where it comes
from exactly. However, switching to the 'select'-based selector solves
the problem. As we do not really need the performance improvements
promised by 'poll', it is ok to always use 'select' in redis-py.

Globals from the past

Avoid code duplication

added members initialization

This implies modifying a lot of counters code

Also, register motor controller in `controllers` category

common/axis.py: moved SoftAxis code into its own file

Solves module import problem

Only where it makes more sense, ie. Axis objects and Lima objects

Something more general has to be done. The idea is to make a real
'reload' feature.

* use main.__dict__ as env dict by default
* add config object and standard functions in Session

Fix flint curve display

Closes #717 and #718

See merge request !1270