Add user function in rxode2 ui that can change code when evaluating

[mattfidler]

Fore example

linMod(x, 3)

in the code base would translate to:

rxLM1_1 + rxNM1_2*x + rxNM1_3*x^2 + + rxNM1_4*x^4

And all the new constants would be in the ini file after evaluating the function.

[mattfidler]

This requires:

• a way to register a user function that takes n arguments and changes output code
• a way to push a new $param to the parser (error with overlapping state, lhs, params already defined)
• a way to push population/bsv parameter estimates to the queue to integrate on evaluation of the ui (or ignoring if the model is a "simple/classic" rxode2 model)
• A call to generate code given it is the nth call to the function within the model.

[mattfidler]

For this to be more useful it also will require:

• variable number arguments
• named arguments
• lhs of the current expression

And in conjunction with nlmixr2est would require:

• A user function that can see the data to be fit/simulated.