Add sparse low order SBP operators

[codecov[bot]]

Codecov Report

Merging #82 (9f83024) into main (8bd9709) will decrease coverage by 0.71%. The diff coverage is 67.12%.

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
- Coverage   97.17%   96.47%   -0.71%     
==========================================
  Files          25       25              
  Lines        2760     2834      +74     
==========================================
+ Hits         2682     2734      +52     
- Misses         78      100      +22     

Impacted Files	Coverage Δ
src/StartUpDG.jl	100.00% <ø> (ø)
src/RefElemData_SBP.jl	84.57% <67.12%> (-11.08%)	:arrow_down:

... and 2 files with indirect coverage changes

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