Add gradient measurement method to the BaseProfile class - rather than doing it hodge-podge for classes that need it (hydrostatic mass)

[DavidT3]

This would be much neater, and the gradient of any profile is going to tell you something interesting, even if it is only really required for putting together the hydrostatic mass profile.

Would also be a much neater way of dealing with the 'is there are model or is it just data to be interpolated' aspect of the 'mass()' method of the new hydro mass class (see #1260).