Enforce scalar eltype of mesh vertices

[pablosanjose]

Closes #75

This errors informatively if we try to make a marchingmesh of non-Number vertex eltype, and enforces the constraint at the MeshSpec and Mesh levels.

[codecov-commenter]

Codecov Report

Merging #101 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   62.02%   62.04%   +0.01%     
==========================================
  Files          16       16              
  Lines        2652     2653       +1     
==========================================
+ Hits         1645     1646       +1     
  Misses       1007     1007              

Impacted Files	Coverage Δ
src/mesh.jl	93.33% <100.00%> (+0.04%)	⬆️

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