Implementation of the log of the lower incomplete gamma function

[JacobHass8]

See #1346 and #1173.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.29%. Comparing base (1d670c8) to head (9abdc80).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1349   +/-   ##
========================================
  Coverage    95.29%   95.29%           
========================================
  Files          814      814           
  Lines        67422    67422           
========================================
  Hits         64249    64249           
  Misses        3173     3173           

Files with missing lines	Coverage Δ
include/boost/math/special_functions/gamma.hpp	99.85% <ø> (ø)

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[jzmaddock]

I think your added series is the same as the existing:

math/include/boost/math/special_functions/gamma.hpp

Line 425 in 1d670c8

BOOST_MATH_GPU_ENABLED inline T lower_gamma_series(T a, T z, const Policy& pol, T init_value = 0)

[JacobHass8]

I think your added series is the same as the existing:

math/include/boost/math/special_functions/gamma.hpp

Line 425 in 1d670c8

BOOST_MATH_GPU_ENABLED inline T lower_gamma_series(T a, T z, const Policy& pol, T init_value = 0)

You're absolutely right. Based on the name of the function, I assumed it was a continued fraction implementation. This makes things a lot easier!

[JacobHass8]

I also changed the criteria for when the asymptotic series is used based on code paths in previous functions that used the asymptotic series. With my limited spot testing, this gives accurate results and is ready for testing.