Use AST module to help post-process assignments in scripts

[mwtoews]

The primary changes in this PR is with scripts/process-scripts.py where the ast module is used to identify assignment expressions in the scripts to gather information for the docs. Some of these assignments may span several lines. For instance:

k33_str = (
    "1179., 30., 30., 30., 30."  # Vertical hydraulic conductivity ($ft/d$)
)

will extract the assignment string 1179., 30., 30., 30., 30. and comment Vertical hydraulic conductivity ($ft/d$)

The new refactor will also allow more than one table, however it doesn't seem this feature is currently used.

A minor limitation is that ast.unparse requires Python 3.9+, which is no issue for CI which already uses 3.9.

Other changes to *.ipynb files are minimal updates to sync with the *.py scripts, using the --update option.

[langevin-usgs]

Tables were a little fragile, and it is nice to have the option to have multiple tables, so this looks good to me.