Support "percent" as a unit in Taurus

[cpascual]

current pint (v0.8) does not define a unit for percent.
This could be added in taurus.external.pint. See the following recipe:

The recipe is to do :

In [1]: from taurus.external.pint import UR, Q_
In [2]: UR.define('percent = 0.01*count = %')
In [3]: print Q_("50 percent") 
50 %

In [4]: print Q_("50 percent").to('dimensionless')
0.5

In [5]: print Q_("50 percent") + 1
1.5

BUT: the % symbol is not supported in pint yet (see hgrecco/pint#429 ).

So I see three possibilities:

• wait for the above issue in pint to be resolved before introducing this in taurus.
• register the percent without the abbreviation: UR.define('percent = 0.01*count')
• register the percent with pct as abbreviation instead of % (but this is IMHO problematic because once "%" is supported we would introduce a backwards-incompatibility if we wanted to switch to %)

[cpascual]

I removed the milestone since I think that we should wait for this to be solved in hgrecco/pint#429