Implement concepts

[mborland]

Should significantly improve error messages as discussed in: boostorg/math#848

Closes: #68

[codecov]

Codecov Report

Merging #347 (ea1a1e1) into develop (beb2ad6) will not change coverage.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #347   +/-   ##
=======================================
  Coverage     97.6%   97.6%           
=======================================
  Files           73      73           
  Lines         3346    3346           
=======================================
  Hits          3265    3265           
  Misses          81      81           

Files	Coverage Δ
include/boost/decimal/decimal128.hpp	95.6% <ø> (ø)
include/boost/decimal/decimal32.hpp	99.9% <100.0%> (ø)
include/boost/decimal/decimal64.hpp	98.6% <ø> (ø)
include/boost/decimal/detail/cmath/abs.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/cmath/acosh.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/cmath/asinh.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/cmath/atanh.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/cmath/ceil.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/cmath/cos.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/cmath/cosh.hpp	100.0% <ø> (ø)
... and 36 more

---

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 beb2ad6...ea1a1e1. Read the comment docs.