Add-custom-module-functions-functionality

[mleot]

Motivation:

I would like to be able to specify custom function in my codebase (which returns a pybamm compatible function) in a BPX json file by passing the called function name and any arguments.

Example:

"OCP [V]": "mypackage.mymodule.myfunction('arg1','arg2')"

In this case, an import should be added to the preamble, for the package name, and then the function should not be treated as a mathmatical expression.

TODO:

• Find the right way to recognize that the function is this type of function, and not the mathmatical expression string. Currently, I have looked to see if '"' exists in the string (a string argument will not normally appear in the mathmatical expression version of a BPX function).