Fix scalar division for Octonions and add testing

[mborland]

Closes: #1374

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.34%. Comparing base (e6fb7d3) to head (0074609).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1375   +/-   ##
========================================
  Coverage    95.34%   95.34%           
========================================
  Files          825      825           
  Lines        68160    68160           
========================================
+ Hits         64986    64987    +1     
+ Misses        3174     3173    -1     

Files with missing lines	Coverage Δ
include/boost/math/octonion.hpp	70.76% <ø> (ø)

... and 1 file with indirect coverage changes

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