SeismicLookupTable: Enable construction from data

[anowacki]

Add several arguments to the SeismicLookupTable constructor, one for each property read from a table, allowing one to create an instance without having to read files from disk. Passing a single argument still reads a file from disk, however, meaning this change is backwards-compatible.

Along the way, tidy up the internal state held by SeismicLookupTable:

• Remove .P and .T. This requires changes in:
  • plot_table and
  • plot_table_contour.
• Remove .table as once we have used this when reading a file we no longer need to retain it in memory.
• Remove .n_uniq_p and .n_uniq_t as these were unused.
• Remove .pstep as this was only used for converting the data read from a file
• Rename things to be all lowercase.

Pull Request Checklist. Please read and check each box with an X. Delete any part not applicable.

Describe what you did in this PR and why you did it.

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.