Validates key dimension input

        Returns kdims if no dimensions are specified

def _valid_dimensions(self, dimensions):
        """Validates key dimension input

        Returns kdims if no dimensions are specified"""
        if dimensions is None:
            dimensions = self.kdims
        elif not isinstance(dimensions, list):
            dimensions = [dimensions]

        valid_dimensions = []
        for dim in dimensions:
            if isinstance(dim, Dimension): dim = dim.name
            if dim not in self.kdims:
                raise Exception("Supplied dimensions %s not found." % dim)
            valid_dimensions.append(dim)
        return valid_dimensions