Rework scalar multiplication and division

[jwallwork23]

Towards #158.
(Refined from #286.)

This PR revises the scalar multiplication/division approach as described on #286:

I spent far too much time trying to get the gradients to be computed correctly for scalar multiplication and division as it is currently implemented. However, I discovered that everything works nicely if we just define scalars as rank-1 tensors with a single value. The revised approach to scalar multiplication/division adds a bit of boilerplate, but perhaps this could be reduced by introducing a torch_scalar class (see #285 for discussion).

There are lots of deletions because several fypp sections are no longer needed.

The PR also adds some info on this to both the autograd doc page and the autograd example.