Thermal Shallow-Water Test Cases

[tommbendall]

This PR provides some test cases for the thermal shallow-water equations:

1. the thermal Williamson 5 test
2. the thermal Galewsky jet test
3. a spherical gravity wave in a moist and cloudy atmosphere

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Add or update a Gusto case study

Here is a checklist of things that should be done to add a new case study to
the repository:

• The case study has been prepared from the case studies template in templates/template_case_study.py. This ensures that the case study:
  • begins with documentation of the case
  • includes a dictionary of default argument values
  • is run through a function
  • follows the standard order of sections:
    1. test case parameters
    2. set up of model objects
    3. initial conditions
    4. run
  • includes a __main__ routine with arg-parsing of command line arguments
• The case study has a quick-to-run test form in the relevant test_*.py file, so that it will be run as part of CI
• A plotting script has been added to the relevant plotting directory, with a name that matches the case study script
• Neat figures have been added to the relevant figures directory, with names that match the case study script

Add or update a plotting script

Here is a checklist of things that should be done to add a new plotting script to the repository:

• The plotting script has been prepared from the template in templates/template_plotting_script.py or another acceptable plotting script
• The plot follows the Good Plot Guide in tomplot/good_plot_guide.md
• Relevant initial and final fields are plotted
• The figures produced have been added to the repository