Add matrices

[agdestein]

This adds a few matrices, including

• boundary conditions
• divergence
• pressuregradient
• diffusion

The old laplacian_mat is simplified,
and time steppers for implicit diffusion can now maybe use the diffusion matrix directly.

[codecov]

Codecov Report

Attention: Patch coverage is 86.54971% with 46 lines in your changes missing coverage. Please review.

Project coverage is 61.05%. Comparing base (48da56d) to head (d099776).
Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
src/matrices.jl	85.66%	45 Missing ⚠️
src/operators.jl	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #125      +/-   ##
==========================================
+ Coverage   59.39%   61.05%   +1.65%     
==========================================
  Files          30       30              
  Lines        3699     3941     +242     
==========================================
+ Hits         2197     2406     +209     
- Misses       1502     1535      +33     

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