Add cmath nan function

[mborland]

Closes: #342

[codecov]

Codecov Report

Merging #343 (89e4065) into develop (a1956a6) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #343   +/-   ##
=======================================
  Coverage     97.6%   97.6%           
=======================================
  Files           73      73           
  Lines         3346    3346           
=======================================
  Hits          3265    3265           
  Misses          81      81           

Files	Coverage Δ
include/boost/decimal/cmath.hpp	100.0% <ø> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a1956a6...89e4065. Read the comment docs.