add generic fallback for gamma_inc

[araujoms]

Instead of having a specific method for BigFloat, this uses a generic fallback. Not only it simplifies existing code, but also it makes gamma_inc work for generic types from Quadmath, DoubleFloats, MultiFloats.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.28%. Comparing base (ea68a71) to head (c43542a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #528      +/-   ##
==========================================
- Coverage   94.28%   94.28%   -0.01%     
==========================================
  Files          14       14              
  Lines        3026     3025       -1     
==========================================
- Hits         2853     2852       -1     
  Misses        173      173              

Flag	Coverage Δ
unittests	94.28% <100.00%> (-0.01%)	⬇️

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