For backward-compatibility, I think it makes sense to conitnue to allow users to use fixed with no depth weighting, even if it's not ideal. It should work to make users always pass a loose forward to make_inverse_operator, and convert if needed. 

Having a `force_fixed` option is redurndant with `loose=None`, but it makes sense to have that redundancy if we want to discourage users from using a fixed-orientation solution.