Update base_convert to set a single object

[psychemedia]

Currently, base_convert sets several model parameters separately:

base_convert(self, input_text, output_template="", force=False, params=None):
        self.set_output_template(output_template)
        self.params = params if params else {}
        self.set_doc_content(input_text, force=force)

Should this all be in a single dict? Actually, a tuple, so we can unpack it in one go?

In a non-blocking route, could there be races in reading and/or changing things? Could there be races anyway?

We could also have a msg ID in the dict; incrementing this for each convert request should force a new model update too? The response object could then also include the msg ID.

What happens if in non-blocking there are several convert requests on the py side? Can we lose messages on the js side? Eg if there is an onchange in the dict, does the handler get a reference to the dict? If there are two rapid onchanges, could the first onchange handler end up reading the dict values that created the second onchange event?