codegen baseline

[mjreno]

Continuation of #176.

Introduces a code generation utility that produces Python package classes directly from MODFLOW 6 DFN specification files. Currently integrates unsupported packages (python package file not in repo), manually maintained package files and codegen generated files.

Workflow:

# Regenerate all existing, codegen controlled generated files from the develop branch DFNs
pixi run generate-classes

# Preview generation into a temp directory without touching the repo
pixi run generate-classes-preview

# Target a specific upstream ref or local DFN directory
generate-mf6-classes --ref 6.6.0
generate-mf6-classes --dfnpath /path/to/modflow6/doc/mf6io/mf6ivar/dfn

This PR extends support to a limited set of GWF/GWT/GWE/PRT model packages and is intended to be a step towards broad coverage.