clip fails for *dimensionless* Quantities when the limits are not Quantities. #108

The current implementation of clip tests whether a_min and a_max are Quantities, except for the case of dimensionless quantities. I assume that the idea is to allow to clip dimensionless quantities with limits that are not quantities itself. However, the implementation then goes on to rescale the limits to the dimensionality of the main argument, which of course fails, as the limits might not have a method .rescale.