Add missing tests

[sethaxen]

The goal of this PR is to get (close to) complete coverage by adding missing tests. for already-implemented functionality.

[codecov]

Codecov Report

Merging #74 (a779f44) into master (247d149) will increase coverage by 38.18%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master      #74       +/-   ##
===========================================
+ Coverage   58.94%   97.12%   +38.18%     
===========================================
  Files           3        3               
  Lines         397      383       -14     
===========================================
+ Hits          234      372      +138     
+ Misses        163       11      -152     

Impacted Files	Coverage Δ
src/DualQuaternion.jl	95.23% <100.00%> (+73.45%)	⬆️
src/Octonion.jl	96.15% <100.00%> (+68.10%)	⬆️
src/Quaternion.jl	98.50% <100.00%> (+10.18%)	⬆️

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[sethaxen]

This is ready for review now. A few notes:

• It replaces convert methods with more constructors, which allows us to use the default convert methods in base.
• It deletes code for branches that are unreachable by the tests
• A number of tests are broken; these indicate bugs that should be fixed in future PRs.
• All lines should be covered now except for the show methods, which are never used and do not overload Base.show

[hyrodium]

Most seem to be fine. I've added suggestions for adding more tests and minor updates.

[sethaxen]

Thanks for the very thorough review!

[hyrodium]

Added a very minor suggestion.