Better documentation for Operator input/output types

[valeriabarra]

We should have a better documentation of the operators including information of what type of vectors they need for input/output arguments and boundary conditions, e.g., when you use the SetDivergence boundary type in a DivergenceC2F operator, you should feed it a scalar, because you are effectively setting out the operator's output type for that boundary, not the type of input for that operator on the boundary (i.e., a vector).