MultiParameter usability issues

Just some notes of various issues that we should improve with respect to MultiParameters

- [ ] If you provide setpoints but not setpoint_names the setpoints are not used in plots. But stored in set_array in the dataset. Should perhaps raise
- [ ] The documentation claims that setpoints can be a tuple of tuples of various types including numpy arrays. This is currently not true. The code checks that the inner type is among `nt, DataArray, collections.Sequence, collections.Iterator`which `numpy.ndarray` is not. If you add it to the check it will fail with a type error (see below). I have not tested if DataArrays work correct. Converting to a tuple solved the immediate problem
- [x] Missing setpoint_units
- [ ] The multidimensional setpoints are too complicated to use (1D then 2D the 3D etc)

Start by locating the MultiParameter implementation and the paths handling setpoints, setpoint_names, setpoint_units, plotting, and dataset storage. Reproduce the documented tuple and numpy-array cases, then define and test the intended behavior for each unchecked item, including whether invalid combinations should raise and whether multidimensional setpoints need a redesigned interface.

Written by the indexing model from the issue text.