checkbox to allow automatic conversion to enumerated string

[prjemian]

• FIX BUG: Conditional visibility calculations don't display correctly in converted MEDM screens #760
• ref apply use_enum to enforce MEDM visibility calcs use default data type BCDA-APS/adl2pydm#84 waiting on this PR
• ref "Connected" text does not show BCDA-APS/adl2pydm#49
• ref some screens showing rectangles visible by dynamic attribute BCDA-APS/adl2pydm#81

This PR adds a Enum? checkbox in the rules for a widget associated with a use_enum property for each channel. When checked/True, which is the default behavior now, an attempt will be made to use the channel's value as an index into the list of enumerated strings (enum_strs) for the channel, converting the number to text. Since MEDM screens do not make this conversion, this PR allows the conversion to be an option, controlled by the checkbox and use_enum property.

The adl2pydm converter will add support for this use_enum property, setting it to False for all MEDM dynamic attribute calculation channels.

[codecov-commenter]

Codecov Report

Merging #762 (863307f) into master (e0996a5) will increase coverage by 0.02%.
The diff coverage is 68.96%.

@@            Coverage Diff             @@
##           master     #762      +/-   ##
==========================================
+ Coverage   58.55%   58.57%   +0.02%     
==========================================
  Files          88       88              
  Lines       10111    10123      +12     
==========================================
+ Hits         5920     5930      +10     
- Misses       4191     4193       +2     

Impacted Files	Coverage Δ
pydm/widgets/rules.py	70.87% <0.00%> (-0.08%)	⬇️
pydm/widgets/rules_editor.py	96.38% <100.00%> (+0.10%)	⬆️
pydm/widgets/base.py	90.54% <0.00%> (-0.23%)	⬇️

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[mattgibbs]

Hi @prjemian, I'm planning on merging this one - I am polling our users at SLAC to figure out if we should keep the Enum? == True default, or if we should revert back to the old (and in my opinion, better) behavior where the integer index was used.

[prjemian]

@mattgibbs Thanks!