Add erf and erfc

[mborland]

Closes: #356
Closes: #357

[codecov]

Codecov Report

Merging #358 (536a564) into develop (b6092e9) will increase coverage by 0.1%.
The diff coverage is 94.9%.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #358     +/-   ##
=========================================
+ Coverage     97.7%   97.8%   +0.1%     
=========================================
  Files           74      76      +2     
  Lines         3370    3529    +159     
=========================================
+ Hits          3292    3449    +157     
- Misses          78      80      +2     

Files	Coverage Δ
include/boost/decimal/cmath.hpp	100.0% <ø> (ø)
.../decimal/detail/cmath/impl/evaluate_polynomial.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/cmath/erf.hpp	94.7% <94.7%> (ø)

... and 4 files with indirect coverage changes

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