Advanced Nonlinear Laplacian handling

[xtalax]

Extend the nonlinear handling to deal with additional derivs inside the first

Further extension may enable decent mixed derivatives handling, tests needed.

Testing against half ice dome at present

[codecov]

Codecov Report

Merging #291 (5eba37b) into master (c360747) will decrease coverage by 30.02%.
The diff coverage is 86.48%.

@@             Coverage Diff             @@
##           master     #291       +/-   ##
===========================================
- Coverage   81.56%   51.55%   -30.02%     
===========================================
  Files          36       35        -1     
  Lines        1660     1676       +16     
===========================================
- Hits         1354      864      -490     
- Misses        306      812      +506     

Files Changed	Coverage Δ
src/system_parsing/pde_system_transformation.jl	36.11% <ø> (-21.12%)	⬇️
...schemes/nonlinear_laplacian/nonlinear_laplacian.jl	86.53% <82.14%> (-13.47%)	⬇️
...schemes/spherical_laplacian/spherical_laplacian.jl	85.18% <100.00%> (-7.13%)	⬇️
src/system_parsing/interior_map.jl	85.35% <100.00%> (-6.32%)	⬇️

... and 29 files with indirect coverage changes

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