fields with leading underscore

Fields starting with underscore have a special meaning in Pydantic and cannot be supported out of the gate

Things to fix / take care of / ideas:

* Generator should issue a warning when it sees a field with leading underscore or a field that's not valid in Python for some reason
* `GelModel` machinery should mangle the name so that pydantic can have this field at all
* `GelModel` machinery should add an `alias` arg to `FieldInfo` with the original unmangled field name. That will make pydantic accept the original field from input.
* We can also potentially create computeds with demangled names that can write and read from mangled field. Test this.