Changed to auto convert from aux to dim coord

Currently iris raises an error if you try and pass an aux coord to a function that wants a dim coord, even if the aux coord is capable of being converted to a dim coord (i.e. monotonic). This PR attempts to convert aux coords and then raises the current error if that fails.

Note I haven't updated the documentation - not sure what to say: "or an aux_coord that can be converted using DimCoord.from_cube()" or something?