Adding Period_prior_to_index function

[Jay-sanjay]

This PR is related to #1

PR CheckList

• Added the very first dispatch for period_prior_to_index function
• Added the test-suite for the same
• Added the Doc-strings
• Added the remaining dispatches with test-suite

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1d5c06c) to head (6b1e001).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff            @@
##               dev       #35   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         2           
  Lines           16        37   +21     
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+ Hits            16        37   +21     

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