Fix for #71

[tpoisot]

Closes #71

This adds a new convert method (and finally adds a show method) to change the element type of a layer. Useful to plot non-float things.

[codecov-io]

Codecov Report

Merging #72 (96fbf4c) into master (d714156) will decrease coverage by 0.60%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
- Coverage   85.30%   84.69%   -0.61%     
==========================================
  Files          22       22              
  Lines         483      490       +7     
==========================================
+ Hits          412      415       +3     
- Misses         71       75       +4     

Flag	Coverage Δ
unittests	84.69% <50.00%> (-0.61%)	⬇️

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Impacted Files	Coverage Δ
src/lib/overloads.jl	90.62% <42.85%> (-3.76%)	⬇️
src/recipes/recipes.jl	93.10% <100.00%> (ø)

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