Adding Indices calculation

[chiaral]

This will try to address issues #13 #14 #15

[codecov-io]

Codecov Report

Merging #18 into master will decrease coverage by 13.15%.
The diff coverage is 5.81%.

@@             Coverage Diff             @@
##           master      #18       +/-   ##
===========================================
- Coverage   61.78%   48.63%   -13.16%     
===========================================
  Files           7        8        +1     
  Lines         280      366       +86     
  Branches       76       84        +8     
===========================================
+ Hits          173      178        +5     
- Misses         83      164       +81     
  Partials       24       24

Impacted Files	Coverage Δ
xcape/core.py	65.07% <11.11%> (-12.7%)	⬇️
xcape/Indices_calc.py	2% <2%> (ø)

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