Implement special function log incomplete gamma function

[JacobHass8]

Implementation of log incomplete gamma function using asymptotic approximations where the incomplete gamma function underflows. See #1173 and #1338.

[JacobHass8]

@jzmaddock I've tried to add error checking and the promotion boilerplate based on your comment #1338 (comment). Does this look okay for a start?

[jzmaddock]

Thanks @JacobHass8 that looks good to me. BTW the separation between lgamma_incomplete_imp_final and lgamma_incomplete_imp shouldn't be needed in this case: that was a hack Matt introduced for some functions to workaround the lack of recursion support in some GPU contexts, but there's no recursion here, so we should be good :)

Some tests and docs and hopefully this should be good to go! Thanks for this.

[JacobHass8]

Some tests and docs and hopefully this should be good to go! Thanks for this.

What file should I put the tests in, math/tests/test_igamma.hpp? Are the spot checks I've implemented so far sufficient?

[jzmaddock]

drone tests were failing for 128-bit long double platforms, I've updated the test data with values from wolframalpha in case the data was truncated to double precision somewhere along the way.

Also not sure why no github actions are being run - anyone any ideas? @mborland ?

With regard to further tests, the area most likely to fail, would be where the implementation first switches to the log asymptotic expansion, and at higher precision (we should support 128-bit long doubles at least, I'm less concerned about full arbitrary precision just yet).

[jzmaddock]

Ah... I needed to approve the github run: now done!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.29%. Comparing base (f6be8e8) to head (40a0c54).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1346   +/-   ##
========================================
  Coverage    95.28%   95.29%           
========================================
  Files          814      814           
  Lines        67364    67396   +32     
========================================
+ Hits         64191    64223   +32     
  Misses        3173     3173           

Files with missing lines	Coverage Δ
include/boost/math/special_functions/gamma.hpp	99.85% <100.00%> (+<0.01%)	⬆️
test/test_igamma.hpp	100.00% <100.00%> (ø)

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[JacobHass8]

With regard to further tests, the area most likely to fail, would be where the implementation first switches to the log asymptotic expansion, and at higher precision (we should support 128-bit long doubles at least, I'm less concerned about full arbitrary precision just yet).

I've added some documentation but don't know how to generate the html. I tried running b2 in the doc folder but received an error from a different file.

I also added a test for $x=999$ which is right below when the asymptotic expansion is used. Is there anything else that needs to be done before this can be merged?