implement \overline command

[karlwessel]

I implemented the support for the overline command basically by pattern recognition of how the \sqrt implementation works. I can't say I fully understood every line, therefore it would be good if somebody can double check if I missed something. Especially the layout part.

Also added basic tests for parsers and layouts handling of overline.

This fixes issue #79.

[codecov-commenter]

Codecov Report

Base: 79.78% // Head: 80.25% // Increases project coverage by +0.46% 🎉

Coverage data is based on head (c590bf4) compared to base (50f8b4c).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
+ Coverage   79.78%   80.25%   +0.46%     
==========================================
  Files           7        7              
  Lines         470      476       +6     
==========================================
+ Hits          375      382       +7     
+ Misses         95       94       -1     

Impacted Files	Coverage Δ
src/parser/commands_registration.jl	34.78% <ø> (ø)
src/engine/layout.jl	90.07% <100.00%> (+0.47%)	⬆️
src/engine/texelements.jl	81.57% <0.00%> (+1.31%)	⬆️

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[Kolaru]

Thanks a lot for this !

Overall this is very good pattern recognition, thanks in particular for taking time to add tests.

The changes are just minor style and layouting changes.

[Kolaru]

This looks perfect, thanks a lot.