Support 3D `q_space_grid` and revisit `load_nxs`

* `q_space_grid` needs to support 3D grids. * Perhaps `load_nxs` should include a `QGrid` object in its return value. For 1D and 2D grids, additional "width" information may be required. This refactor could replace the current 4×4 `covector` matrix. For a description of the current behavior of `load_nxs`, see comments in #256.