Better Mixed Deriv handling, fix second order time

[xtalax]

No description provided.

[codecov]

Codecov Report

Merging #303 (5e1d463) into master (25928a5) will increase coverage by 3.71%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #303      +/-   ##
==========================================
+ Coverage   82.35%   86.07%   +3.71%     
==========================================
  Files          37       38       +1     
  Lines        1700     1723      +23     
==========================================
+ Hits         1400     1483      +83     
+ Misses        300      240      -60     

Files Changed	Coverage Δ
src/MethodOfLines.jl	100.00% <ø> (ø)
src/discretization/generate_ic_defaults.jl	93.33% <ø> (-0.79%)	⬇️
...discretization/generate_finite_difference_rules.jl	95.45% <100.00%> (+0.45%)	⬆️
...mes/2nd_order_mixed_deriv/2nd_order_mixed_deriv.jl	100.00% <100.00%> (ø)
...schemes/centered_difference/centered_difference.jl	100.00% <100.00%> (ø)
src/system_parsing/pde_system_transformation.jl	92.85% <100.00%> (+32.30%)	⬆️

... and 3 files with indirect coverage changes

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