Make value ref dynamic for selects from repeats

[lognaturel]

#472 added support for selects from repeats but it did not make the value ref dynamic so only fields with the name name could be used.

Why is this the best possible solution? Were any other approaches considered?

This matches the way the label ref is set and is simple to understand.

What are the regression risks?

One risk I thought of is that if itemset_value_ref is in the wrong place, some select cases could end up without a ref set.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests_v1
• run nosetests and verified all tests pass
• run black pyxform to format code
• verified that any code or assets from external sources are properly credited in comments

[codecov-io]

Codecov Report

Merging #493 (f1a9780) into master (87325c5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #493   +/-   ##
=======================================
  Coverage   83.84%   83.85%           
=======================================
  Files          25       25           
  Lines        3690     3692    +2     
  Branches      860      860           
=======================================
+ Hits         3094     3096    +2     
  Misses        452      452           
  Partials      144      144           

Impacted Files	Coverage Δ
pyxform/question.py	93.36% <100.00%> (+0.05%)	⬆️

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[DavisRayM]

LGTM