Add comp_ellint_1

[mborland]

Closes: #463

[codecov]

Codecov Report

Attention: Patch coverage is 71.15385% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 98.6%. Comparing base (d56e394) to head (7cad05b).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #464     +/-   ##
=========================================
- Coverage     98.8%   98.6%   -0.1%     
=========================================
  Files          172     175      +3     
  Lines        11245   11303     +58     
=========================================
+ Hits         11104   11141     +37     
- Misses         141     162     +21     

Files	Coverage Δ
include/boost/decimal/cmath.hpp	100.0% <ø> (ø)
test/test_ellint_1.cpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/cmath/ellint_1.hpp	87.5% <87.5%> (ø)
...lude/boost/decimal/detail/cmath/impl/ellint_rf.hpp	54.9% <54.9%> (ø)

... and 4 files with indirect coverage changes

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