WIP: Coupling

[ranocha]

Cf. #51.

[coveralls]

Coverage increased (+0.4%) to 94.107% when pulling 23abcbc on coupling into 3ec9586 on master.

[codecov]

Codecov Report

Merging #62 into master will increase coverage by 0.38%.
The diff coverage is 97.09%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   94.94%   95.33%   +0.38%     
==========================================
  Files          24       25       +1     
  Lines        3460     3748     +288     
==========================================
+ Hits         3285     3573     +288     
  Misses        175      175

Impacted Files	Coverage Δ
src/filter.jl	58.53% <ø> (ø)	⬆️
src/periodic_operators.jl	96.46% <ø> (ø)	⬆️
src/SummationByPartsOperators.jl	100% <ø> (ø)	⬆️
src/SBP_operators.jl	98.35% <ø> (ø)	⬆️
src/legendre_operators.jl	94.82% <100%> (+2.72%)	⬆️
src/dissipation_operators.jl	97.1% <100%> (ø)	⬆️
src/var_coef_operators.jl	94.2% <100%> (+5.96%)	⬆️
src/general_operators.jl	85.14% <100%> (+5.48%)	⬆️
src/coupling.jl	96.4% <96.4%> (ø)
... and 1 more

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