refactor: self-contained example scripts

[wpbonelli]

• intermediate step toward Self-contained example notebooks modflow6#1348
• remove common/ directory
  • move modeling utilities from common/ to the scripts they're used in
    • todo: de-duplicate Wexler3D solution and curvilinear grid builder, move to flopy or devtools?
  • remove latex utilities from common/, import from modflow-devtools
  • remove plot styles from common/, use flopy.plot.styles
  • remove config module dependency, use environment variables and pytest CLI flags to control behaviors (details in DEVELOPER.md)
• download and cache data files behind the scenes with https://www.fatiando.org/pooch/latest/retrieve.html — makes cloning project or manually downloading files unnecessary
• depend on devtools develop branch
• test in CI with python3.12
• black/isort formatting
• fix plot arrangements (legend placement, etc)
• consistent naming for functions/variables (build_models(), run_models(), scenario(), etc)
• standardize tag for autogenerating tables from variable assignments: "Model parameters"