fix `approximation_type::Polynomial{Gauss}` constructor

[jlchan]

Previously it was returning a more general "default" approximation type, now it returns Polynomial{Gauss}

[codecov[bot]]

Codecov Report

Merging #123 (0ea32b4) into main (0410dc2) will increase coverage by 1.09%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
+ Coverage   96.23%   97.33%   +1.09%     
==========================================
  Files          25       25              
  Lines        2841     2848       +7     
==========================================
+ Hits         2734     2772      +38     
+ Misses        107       76      -31     

Impacted Files	Coverage Δ
src/RefElemData_SBP.jl	97.05% <100.00%> (+15.93%)	:arrow_up:
src/RefElemData_polynomial.jl	100.00% <100.00%> (ø)

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[jlchan]

Windows seems to have spurious failing CI again...