Create a `grid.negate_depth` method to swap the sign of depth fields?

[erikvansebille]

Since the sign/orientation of the depth vector is not standardised in oceanography, users may sometimes need to negate a grid.depth field. See e.g. Parcels-code/virtualship#75. Especially since grid._depth is a private attribute, this is not trivial to users.

It might thus be useful to add a simple grid.negate_depth()method that simply does

def negate_depth(self):
    self._depth = -self._depth