Migrate to Cython 3

[mcflugen]

Description

This pull request is an attempt to migrate to Cython 3.

Highlights:

• Changed the build-system (in pyproject.toml) to use cython version 3.0.0b3 or greater. It seems to have some improvements we could use and, I believe, @sebastien-lenard was wanting to use some of that new functionality in his new routing components.
• cythonized (and parallelized using openmp) functions that calculate gradients, and divergence on raster grids.
• Added some (speed) benchmarks for the new cythonized code.
• Added -fopenmp to the compile options. Note that this this is only added if the WITH_OPENMP environment variable is set. I did this because the default compilers on Mac and Windows do not come with openmp. To use openmp on Mac, I use the conda compilers (I imagine use Homebrew would also work).

@gregtucker and/or @sebastien-lenard maybe you could give this a review?

Checklist - did you ...

• Add a news fragment file entry if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?
• All tests have passed?
• Formatted code with black?
• Removed lint reported by flake8?
• Sucessful documentation built? (if documentation added or modified)

[gregtucker]

Looks good to me. Great that you were able to squeeze some speed out!