Ad functions should deal with Ad operators

[keileg]

[HIGHLY TECHNICAL]
Currently functions in pp.ad.functions cannot be applied directly when defining an Ad expression, but must instead be wrapped in a general pp.ad.Function (capital F). This intermediate step seems unnecessary, and should be dropped.

Note to self: A possible solution is to define a decorator which deals with the case of the input argument being an operator.

[vlipovac]

Implementation of decorator is available at

porepy/src/porepy/numerics/ad/operator_functions.py

Line 431 in 88adf95

class ADmethod:

What is missing is an overload of __call__ to return a direct evaluation of the decorated function if called with arguments which are not operators.