Revert "Make first steps towards generalised Dask 'VDS'" because a Dask
'VDS' requires open file handles as the source input, and the previous
approach neglected this.

Co-authored-by: Markus Gerstel <markus.gerstel@diamond.ac.uk>

# Conflicts:
#	src/tristan/make_single_image.py
#	src/tristan/make_vds.py

A precursor to a Dask alternative to VDS, reusing much of the VDS code.

Tidy up the VDS code to rationalise the operation of some functions.
Return VDS-specific code to the VDS module, since it seems to be of
little use in constructing the Dask alternative.

There is a regression in dask.array.bincount that prevents slicing, see
dask/dask#7361.  This has been fixed on dask:main and the fix will
likely be included in a release soon.

We don't use the event_energy data set for anything (yet).

- If missing an input NeXus file, prompt the user.
- If the output file already exists, open in "w" or "x" mode as per user
  instruction.

These seem to have got missed in a mangled merge resolution.

Exploit the trivial broadcasting of divmod over NumPy and Dask arrays to
simplify the function definition and doc.

This comes at the cost of some code clarity because it's been done in a
bit of a hurry!

- Move command-line tools to a dedicated command_line submodule.
- Use argparse subparsers for the various different image binning
  commands.

Use flatnonzero in place of argwhere.

This gets done for all the images command-line programs, so code reuse
makes sense.