Implement Legendre

[mborland]

Closes: #444

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.6%. Comparing base (a7bba37) to head (5803950).
Report is 19 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #446     +/-   ##
=========================================
+ Coverage     98.6%   98.6%   +0.1%     
=========================================
  Files          164     166      +2     
  Lines        10920   10952     +32     
=========================================
+ Hits         10766   10798     +32     
  Misses         154     154             

Files	Coverage Δ
include/boost/decimal/cmath.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/cmath/legendre.hpp	100.0% <100.0%> (ø)
test/test_legendre.cpp	100.0% <100.0%> (ø)

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