Allow users to control what processes are run

[mcflugen]

This pull request adds the ability for a user to control what processes used in a Sequence simulation as well as the order they are run in. This can be controlled with the processes key in the sequence.toml file. For example,

[sequence]
processes = [
  "submarine_diffusion",
  "sea_level",
  "subsidence",
  "flexure",
  "compaction",
]

The process names correspond to the table names used in sequence.toml that give initialization parameters. The following will turn all processes off,

[sequence]
processes = []

but leaving out the processes key completely will turn all processes on. I've done this to maintain backward compatibility with older input files that didn't use the processes key.

I've also improved some of the documentation and began adding type hints.

I've also cleaned up the grid section of the input file. It's been simplified to look like,

[sequence.grid]
n_cols = 100
spacing = 1000.0

instead of the much more verbose and error prone,

[sequence.grid]
shape = [3, 100]
xy_spacing = 1000.0
xy_of_lower_left = [0.0, 0.0]

[sequence.grid.bc]
top = "closed"
bottom = "closed"