GPU Batch 13

[mborland]

This PR resumes work on special functions and adds support for: expint, gegenbauer, gegenbauer_prime, gegenbauer_derivative, cyl_hankel_1, cyl_hankel_2, sph_hankel_1, sph_hankel_2, and hermite.

In the docs I added a note that states in the NVCC or NVRTC case the return type of cyl and sph hankels is in namespace cuda::std:: instead of std:: so you get a cuda::std::complex<T> as the result. Much like other utilities developed so far boost::math::complex<T> is just an alias to the proper type depending on context.

On device CI runs can be found: cppalliance/cuda-math#27

CC: @dschmitz89, @izaid, @steppi

[codecov]

Codecov Report

Attention: Patch coverage is 96.55172% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.78%. Comparing base (c3afa49) to head (9f21419).
Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/math/special_functions/hankel.hpp	92.59%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1201   +/-   ##
========================================
  Coverage    93.78%   93.78%           
========================================
  Files          657      657           
  Lines        53899    53903    +4     
========================================
+ Hits         50551    50555    +4     
  Misses        3348     3348           

Files with missing lines	Coverage Δ
include/boost/math/special_functions/bessel.hpp	100.00% <ø> (ø)
include/boost/math/special_functions/expint.hpp	95.41% <100.00%> (+0.06%)	⬆️
include/boost/math/special_functions/hermite.hpp	100.00% <100.00%> (ø)
test/test_expint.cpp	100.00% <ø> (ø)
test/test_expint.hpp	100.00% <ø> (ø)
test/test_hankel.cpp	100.00% <ø> (ø)
test/test_hermite.cpp	93.75% <ø> (ø)
test/test_hermite.hpp	100.00% <ø> (ø)
include/boost/math/special_functions/hankel.hpp	94.11% <92.59%> (+0.11%)	⬆️

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