Implement Divisor Witness as per section 3.1.1 or 3.1.2

[feltroidprime]

Given a principal divisor (degree = 0), one needs to find an element of the function field interpolating all its points.
See section 3.1.

Infunction_field.py, implement algo 3.1.1 and 3.1.2 that returns a FunctionFelt object from a principal divisor.