Seismic property calculation from composition histograms and lookup tables

[anowacki]

This PR adds the ability to calculate the seismic properties of a model by using its composition histogram and a set of thermodynamic lookup tables.

The merge commit which brings this back into main has fixed a number of errors introduced since this branch forked off main. There are still a number more to be fixed in a future commit.

Closes #42.

• WIP Add lookup table support to TerraModel
• Remove unused import from lookup_tables test
• terra_model: Fix capitalisation of density key
• lookup_tables: Rename _ani variables with _an suffix
• terra_model: Fix TerraModel constructor with MultiTables
• lookup_tables: Place all the property names in a module-global variable
• terra_model: Test loading of lookup tables in TerraModel
• TerraModel: Add surface_radius argument and radius-depth conversion
• TerraModel: Add pressure function and pressure evaluation method
• TerraModel: WIP Implementing evaluation from lookup tables
• TerraModel: WIP Implementing evaluation from lookup tables
• lookup_tables: Fix bug in _harmonic_mean
• lookup_tables._check_bounds: Use numpy.clip and issue warnings
• lookup_tables.plot_table: Remove unnecessary print statement
• lookup_tables: Use more flexible interpolation for tables
• plot.layergrid: Only allow coastlines via cartopy, not not at all
• terra_model.plot_layer: Actually pass on all keyword arguments
• TerraModel: Change pressure units to Pa from GPa
• Fix typo in docstring of angular_distance
• geographic: Add angular_step to compute final position from azimuth and distance
• lookup_tables: Allow MultiTables construction with SeismicLookupTables
• Fix formatting for geographic
• Fix formatting of lookup tables test
• terra_model: Add has_lookup_tables and get_lookup_tables

For all pull requests:

• [x] I have run the contrib/utilities/indent script from the main directory to indent my code.

For new features/models or changes of existing features:

• [x] I have tested my new feature locally to ensure it is correct.
• [x] I have created a testcase for the new feature/benchmark in the tests/ directory.
• [x] I have added a changelog entry in the doc/changes directory that will inform other users of my change.