Implement Money Flow Index

[tsubus]

https://www.investopedia.com/terms/m/mfi.asp

[codecov]

Codecov Report

Merging #36 (9ba295a) into main (a2f4ef8) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   98.32%   98.38%   +0.06%     
==========================================
  Files          41       42       +1     
  Lines         598      621      +23     
==========================================
+ Hits          588      611      +23     
  Misses          7        7              
  Partials        3        3              

Impacted Files	Coverage Δ
indicator_money_flow.go	100.00% <100.00%> (ø)

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