Add allow-mock-accuracy parameter

[seadowg]

Needed for getodk/collect#4768

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

What are the regression risks?

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

Before submitting this PR, please make sure you have:

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

[lognaturel]

The required dependency update has been merged.

[codecov-commenter]

Codecov Report

Merging #560 (d0c6815) into master (3380599) will increase coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #560      +/-   ##
==========================================
+ Coverage   83.31%   83.36%   +0.04%     
==========================================
  Files          25       25              
  Lines        3728     3738      +10     
  Branches      872      875       +3     
==========================================
+ Hits         3106     3116      +10     
  Misses        482      482              
  Partials      140      140              

Impacted Files	Coverage Δ
pyxform/xls2json.py	76.31% <100.00%> (+0.34%)	:arrow_up:

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