Add type parameter to `Polynomial`

[jlchan]

Also adds constructor for the Polynomial{Gauss} approximation type.

[codecov[bot]]

Codecov Report

Merging #115 (30894ae) into main (447a369) will increase coverage by 0.08%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
+ Coverage   97.10%   97.19%   +0.08%     
==========================================
  Files          25       25              
  Lines        2733     2748      +15     
==========================================
+ Hits         2654     2671      +17     
+ Misses         79       77       -2     

Impacted Files	Coverage Δ
src/MeshData.jl	94.62% <ø> (ø)
src/StartUpDG.jl	100.00% <ø> (ø)
src/RefElemData.jl	95.74% <100.00%> (+2.06%)	:arrow_up:
src/RefElemData_TensorProductWedge.jl	100.00% <100.00%> (ø)
src/RefElemData_polynomial.jl	100.00% <100.00%> (ø)
src/ref_elem_utils.jl	88.88% <100.00%> (-0.25%)	:arrow_down:

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