Allows selecting only the needed variables, reducing the size of the
resulting restart files.

Creates restart files from a certain time-slice of a Dataset.

It's unlikely that a user will want method chaining with an output
method, and returning the Dataset means we get a lot of printed output
if we just call 'ds.bout.to_restarts()'.

- Set data_vars="minimal" in concat() when combining boundary_pad with
  ds - ensures that variables without an x- or y-dimension do not have a
  new dimension added.
- Make boundary_pad a copy of the boundary cells, then we can set to NaN
  instead of using the strange expression with 'where()'.
- Call .load() on both boundary_pad and ds before combining. Ensures
  that concat() with data_vars="minimal" does not fail because of
  different dask-states of the underlying data arrays.

xarray requires less-than-6-months old dask, so 0.16.0 requires
dask-2.10.

Probably more 'correct' and needed to make to_restart() tests work.

Most of checks need to check numbers divisible by MXSUB or MYSUB, not
NXPE or NYPE.

BOUT++ does not require ixseps1 or ixseps2 to be on processor
boundaries, so allow any splitting in x where nxpe divides nx-2*mxg

This is the most common case, so better to test this. Also tests more of
the code as some functions are only used when there are guard cells.
Leaving out no-guard-cell case just to save time on the tests (it's not
commonly used).