Loading a model fitted with TemplateExpressionSpec

[ibengtsson]

Hi!

First of all, thank you for a great library.

I'm having some issues when working with TemplateExpresssionSpec, and it could very well be a fault on my end... If I create a model with a TemplateExpression, fit and save it, and then try and load it (with from_file) in a different script, I cannot do any model predictions. I just get the error:

ValueError: Failed to evaluate the expression. If you are using a custom operator, make sure to define it in `extra_sympy_mappings`, e.g., `model.set_params(extra_sympy_mappings={'inv': lambda x: 1/x})`, where `lambda x: 1/x` is a valid SymPy function defining the operator. You can then run `model.refresh()` to re-load the expressions.

I don't use any custom operators in my TemplateEpxressionSpec, but maybe I still need to add some mappings to make the loading work?

Grateful for any help!

[MilesCranmer]

Unfortunately template expressions aren't yet compatible with loading from file, sorry. I should definitely update the error message to mention this though, so I will move this to an issue.