Financial functions: part 2

[zgordan-vv]

Addresses #356

This change is 

[codecov]

Codecov Report

Merging #360 into master will increase coverage by 0.18%.
The diff coverage is 57.86%.

@@            Coverage Diff            @@
##           master    #360      +/-   ##
=========================================
+ Coverage   54.52%   54.7%   +0.18%     
=========================================
  Files         199     199              
  Lines       16370   16918     +548     
=========================================
+ Hits         8925    9255     +330     
- Misses       6760    6871     +111     
- Partials      685     792     +107

Impacted Files	Coverage Δ
spreadsheet/cell.go	45.82% <100%> (ø)	⬆️
spreadsheet/formula/fnfinance.go	62.17% <57.78%> (-3.13%)	⬇️
spreadsheet/formula/lexer.go	27.22% <0%> (+0.1%)	⬆️
spreadsheet/formula/fndatetime.go	65.39% <0%> (+0.6%)	⬆️

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