Allow string values in "User-defined" parameters

[ejfdickinson]

To extend the functionality of the "User-defined" parameterisation section, it would be useful to allow general string content here, to express metadata about how other parameters in the same section should be interpreted.

Corollary issue: Would there be any way to distinguish such strings from "function as string" specification?

[ikorotkin]

Is there a reason not to allow the "User-defined" section to be anything (a valid JSON in general), including entire JSON branches to specify new models not supported by BPX yet?

I.e., I should be able to use this section to specify any extra parameters I need, e.g.:

"User-defined": {
    "My model 1": {
        "Submodel": "Type I",
        "Parameter 1": 0.1,
        "Coefficients": [0.1, 0.2, 0.3, 0.4],
        ...
    },
    "My model 2": {
        ...
    }
}

[ejfdickinson]

@ikorotkin This recommendation was made by #46. I think it's a good one!

The main issue that I wanted to raise here was that the schema-defined branches of BPX use string constants to encode functional equations. I don't have the insight to know if there is a way to 'signal' when a string is a string, and when a string is to be read 'as an equation'.