Add support for unknown functions

[isuruf]

Python wrappers support using Python functions and using callbacks.

For example, airyai is a function not supported, but Calculus.jl has a list that we can use.

This requires additions to libsymengine.

[houtanb]

@isuruf Thanks for your work on this project. Do you have a rough estimate of when this feature will be ready?

[isuruf]

Do you have a rough estimate of when this feature will be ready?

Not at the moment.