Refactor, separate out schemes, prepare interface for user choice of scheme

[xtalax]

No description provided.

[codecov]

Codecov Report

Merging #132 (358d67d) into master (07d1f6b) will increase coverage by 0.26%.
The diff coverage is 93.22%.

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
+ Coverage   88.29%   88.56%   +0.26%     
==========================================
  Files          13       23      +10     
  Lines         940     1067     +127     
==========================================
+ Hits          830      945     +115     
- Misses        110      122      +12     

Impacted Files	Coverage Δ
src/discretization/generate_bc_eqs.jl	80.85% <ø> (ø)
...discretization/generate_finite_difference_rules.jl	100.00% <ø> (+4.70%)	⬆️
...scretization/schemes/fornberg_calculate_weights.jl	100.00% <ø> (ø)
src/interface/grid_types.jl	100.00% <ø> (ø)
src/system_parsing/bcs/periodic_map.jl	100.00% <ø> (ø)
src/system_parsing/interior_map.jl	92.66% <ø> (ø)
src/MOL_utils.jl	78.10% <58.62%> (-6.03%)	⬇️
src/system_parsing/bcs/parse_boundaries.jl	97.93% <80.00%> (ø)
...ion/schemes/upwind_difference/upwind_difference.jl	87.87% <87.87%> (ø)
...schemes/spherical_laplacian/spherical_laplacian.jl	88.00% <88.00%> (ø)
... and 11 more

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[xtalax]

@ChrisRackauckas This is just a refactor/moving files around, can I merge this? Only new feature is MOLFiniteDifference change