Allow Interface boundary conditions

[xtalax]

Allow connecting 2 variables together in bcs, if they have the same dimensionality and a similar argument signature, differing in no or one variable.

This only works for connecting the lower of one variable to the upper of another

Supersedes Periodic conditions

fixes #32
fixes #202

[codecov]

Codecov Report

Merging #193 (5083def) into master (f449ba1) will decrease coverage by 5.42%.
The diff coverage is 67.97%.

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
- Coverage   80.47%   75.05%   -5.43%     
==========================================
  Files          35       37       +2     
  Lines        1685     1840     +155     
==========================================
+ Hits         1356     1381      +25     
- Misses        329      459     +130     

Impacted Files	Coverage Δ
src/MOL_utils.jl	64.24% <ø> (-2.43%)	⬇️
src/interface/solution/plot_utils.jl	0.00% <0.00%> (ø)
src/system_parsing/pde_system_transformation.jl	40.74% <16.66%> (+0.61%)	⬆️
src/MOL_discretization.jl	63.39% <54.54%> (-12.76%)	⬇️
src/discretization/generate_bc_eqs.jl	68.20% <60.60%> (-14.76%)	⬇️
src/system_parsing/bcs/parse_boundaries.jl	81.53% <72.60%> (-17.38%)	⬇️
src/discretization/interface_boundary.jl	73.13% <73.13%> (ø)
src/discretization/schemes/WENO/WENO.jl	89.47% <85.71%> (-3.51%)	⬇️
...ion/schemes/upwind_difference/upwind_difference.jl	84.05% <86.66%> (-4.18%)	⬇️
...schemes/spherical_laplacian/spherical_laplacian.jl	92.30% <87.50%> (ø)
... and 18 more

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