Implement atan2

[mborland]

Closes: #372

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bb86162) 98.8% compared to head (968a05e) 98.8%.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #374     +/-   ##
=========================================
+ Coverage     98.8%   98.8%   +0.1%     
=========================================
  Files          134     136      +2     
  Lines         9826    9970    +144     
=========================================
+ Hits          9701    9845    +144     
  Misses         125     125             

Files	Coverage Δ
include/boost/decimal/cmath.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/cmath/atan.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/cmath/atan2.hpp	100.0% <100.0%> (ø)
test/test_atan.cpp	100.0% <100.0%> (ø)
test/test_atan2.cpp	100.0% <100.0%> (ø)

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