2nd order time derivatives

[xtalax]

Implements 2nd order time derivatives, still some issues to work out but progressing well.

fixes #150 fixes #52 fixes #76

[codecov[bot]]

Codecov Report

Merging #151 (70e0ca3) into master (f85ac4e) will decrease coverage by 28.36%. The diff coverage is 78.57%.

@@             Coverage Diff             @@
##           master     #151       +/-   ##
===========================================
- Coverage   88.84%   60.48%   -28.37%     
===========================================
  Files          31       30        -1     
  Lines        1390     1402       +12     
===========================================
- Hits         1235      848      -387     
- Misses        155      554      +399     

Impacted Files	Coverage Δ
src/system_parsing/bcs/parse_boundaries.jl	88.69% <71.42%> (-9.33%)	:arrow_down:
src/MOL_discretization.jl	62.01% <100.00%> (-20.80%)	:arrow_down:
src/system_parsing/interior_map.jl	92.75% <100.00%> (-0.73%)	:arrow_down:
...tization/schemes/half_offset_centred_difference.jl	0.00% <0.00%> (-100.00%)	:arrow_down:
src/discretization/schemes/WENO/WENO.jl	0.00% <0.00%> (-92.99%)	:arrow_down:
src/interface/scheme_types.jl	25.00% <0.00%> (-75.00%)	:arrow_down:
...schemes/nonlinear_laplacian/nonlinear_laplacian.jl	38.70% <0.00%> (-61.30%)	:arrow_down:
...hemes/centered_difference/centered_diff_weights.jl	50.00% <0.00%> (-50.00%)	:arrow_down:
src/discretization/schemes/half_offset_weights.jl	51.35% <0.00%> (-48.65%)	:arrow_down:
...n/schemes/upwind_difference/upwind_diff_weights.jl	48.64% <0.00%> (-48.65%)	:arrow_down:
... and 19 more

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[xtalax]

Working pending https://github.com/SciML/ModelingToolkit.jl/issues/1741 for better solution retrieval