FIX: two issues in chebyshev script were fixed

[OmidOftadeh]

Line 66: big_m=big_m for keyword argument
Line 172: isinstance was replaced by issubclass

[codecov]

Codecov Report

Merging #37 into dev will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##              dev      #37   +/-   ##
=======================================
  Coverage   70.83%   70.83%           
=======================================
  Files          46       46           
  Lines        3401     3401           
=======================================
  Hits         2409     2409           
  Misses        992      992

Impacted Files	Coverage Δ
pytfa/analysis/chebyshev.py	0% <0%> (ø)	⬆️

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