save_model extension to save lith_block and custom section grids

[Japhiolite]

Description

This PR includes an extension of gp.save_model to include custom section grids, as well as the lith_block on the regular grid.

Relates to #691

Checklist

• My code follows the PEP 8 style guidelines.
• My code uses type hinting for function and method arguments and return values.
• My code contains descriptive and helpful docstrings
  which are formatted per the Google Python Style Guidelines.
• I have created tests which entirely cover my code.
• The test code either 1. demonstrates at least one valuable use case (e.g. integration tests)
  or 2. verifies that outputs are as expected for given inputs (e.g. unit tests).
• New and existing tests pass locally with my changes.

[Leguark]

Great thanks