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 (4e9f441).

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!