Adds logic to automatically trim empty columns

[chaburkland]

Hi @flexatone - this is a quick sketch to address an issue with xlsx Frame parsing. I don't have an up-to-date SF dev environment, so this is more meant as a proof of concept.

Basically, XLSX worksheets can easily report max_columns as being many more than expected. For example, you might have a worksheet with 3 columns, but ws.max_columns reports 1025, due to formatting. I know there is a nadir option, but it seems strange to support attempting to build impossible frames.

If 1022 columns are simply EmptyCells, then the frame can never construct due to the repeated None's. My thought is to look at the header column, and automatically ignore all trailing EmptyCells.

(I just realized my sketch doesn't account for index hierarchies, but oh well 🙃)

What are your thoughts?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (17ffbe4) to head (98202f6).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #912   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           63        63           
  Lines        21877     21885    +8     
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+ Hits         21877     21885    +8     

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