Type which are convertible to a number should not participate in arit…

[jzmaddock]

…hmetic operator overloads.

Fixes #608

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.1%. Comparing base (3c9ff4d) to head (4fe16b7).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #609     +/-   ##
=========================================
- Coverage     94.1%   94.1%   -0.0%     
=========================================
  Files          274     274             
  Lines        28563   28563             
=========================================
- Hits         26874   26872      -2     
- Misses        1689    1691      +2     

Files	Coverage Δ
...nclude/boost/multiprecision/detail/number_base.hpp	98.0% <ø> (ø)

... and 1 file with indirect coverage changes

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[mborland]

Should we ask Marshall about merging this into master once it's permissible again?

[jzmaddock]

Probably yes.