🔨 Lookback-functions

[serkor1]

📚 What?

This PR introduces unexported lookback-functions for calculating lookback-periods independently of the indicator functions. The goal is to provide an input validation step for downstream wrappers like {ledgr}, and similar packages that may or may not use different control-flows.

📋 TODO

• Test the overhead of calculating lookback (should be O(1))
• Implement unit-tests (attributes vs lookback)
• Implement high-level wrapper talib::lookback(FUN, ...) similar to indicator()

This PR closes #70

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.67%. Comparing base (a2d960c) to head (3764c17).

Additional details and impacted files

@@           Coverage Diff           @@
##            devel      #71   +/-   ##
=======================================
  Coverage   96.67%   96.67%           
=======================================
  Files         280      280           
  Lines       21012    21012           
=======================================
  Hits        20314    20314           
  Misses        698      698           

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