Label strings in `model_variables.json` not rendering properly

[jdebacker]

Some the string labels using for plotting and stored in model_variables.json are not rendering properly because they have escape characters. These include anything with a "tilde", e.g.,:

    "c": {
        "desc": "Household consumption of composite good",
        "section": "Household Variables",
        "type": "array-like",
        "TPI dimensions": "TxSxJ",
        "SS dimensions": "SxJ",
        "label": "Household Consumption ($\tilde{c}_{j,s,t}$)",
        "toGDP_label": ""
    }

And also $\theta.

JSON doesn't allow raw strings, but perhaps these strings can be cast as raw strings when read into constants.py at line 19.