Include functionality for user defined external variables

[FinbarArgus]

It would be extremely useful if we could define an external variable as a string of C++ code (C++ code snippets), like they do in Fenics.

With this we could:

• Use random number generators
• Input data vs time traces as model input

And much more. It would be a good general way to improve flexibility of use before a more robust standards based approach for doing these types of things could be implemented for each new feature.

[agarny]

FTR, this is possible in libCellML, so we "just" need to come with some API that will allow use to do it from libOpenCOR.