Add a helper function _getsubcolumns

[junyuan-chen]

_getsubcolumns is a helper function for obtaining a collection of columns over selected rows and dropping Missing from element types.

[codecov]

Codecov Report

Merging #3 (2591004) into master (39236ca) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master       #3   +/-   ##
=======================================
  Coverage   99.56%   99.56%           
=======================================
  Files           8        8           
  Lines         456      459    +3     
=======================================
+ Hits          454      457    +3     
  Misses          2        2           

Impacted Files	Coverage Δ
src/DiffinDiffsBase.jl	100.00% <ø> (ø)
src/procedures.jl	100.00% <100.00%> (ø)

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