Add CLI option to construct model from stored TOML parameters

[nefrathenrici]

Purpose

This provides a simple example for driving a model using parameters stored in a TOML file.

To-do

Content

• Adds a CLI option for an input TOML
• Changes the case names to match their corresponding parameter structures. This is not necessary, but simplified the case parsing.
• Adds a constructor for the SphericalParameters using a TOML
• Adds a constructor for all other Test structs
• Adds TOML files with defaults for all test cases. This makes up the bulk of the PR, actual code changes are minor.
• For now, using a TOML file requires that all relevant parameters are stored in the TOML - there is no mixing and matching between the default constructor and the TOML constructor. This is basically the simplest working example for TOML-driven models.
• The TOML file can only be used for model parameters, not configurations.

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• [ ] I have read and checked the items on the review checklist.

[simonbyrne]

Looks good!

I'd like to use this as a playground for exploring how we set up drivers and parameters for other components, so a few things to think about:

• what do we want this to look like?
• when should we specify floating point types?
• how should we handle experiment-specific parameters?
• how should we handle common parameters?