Implementation of new mixed boundaries

[jlperla]

After #142

Specializations for both regular and irregular grids, with tests and docs.

• [x] (Mixed(x_1, Forwards), Mixed(xi_2, Backwards))
• [x] (Mixed(x_1), Mixed(xi_2)) -> (Mixed(x_1, Forwards), Mixed(xi_2, Backwards)) i.e., the defaults give the same as that!
• [x] (Mixed(x_1, Forwards), Mixed(xi_2, Forwards))
• [x] (Mixed(x_1, Backwards), Mixed(xi_2, Backwards))

I think that is enough. We don't need combinations of (forwards, default) or (backwards, forwards) or whatever