Remove hardcoded single-threaded dask scheduler for dataset combine

[dougiesquire]

Change Summary

With this PR, dataset combine is no longer hardcoded to use a single-threaded dask scheduler. Hardcoding this, leads to regular UserWarnings as many/most users already have an active distributed client. I'm happy to be told that this is a bad idea, or that there's a better way to fix the issue reported in #596, but I'm submitting this now to get the ball rolling.

Related issue number

Closes #596

Checklist

• [x] Unit tests for the changes exist (not really appropriate here)
• [x] Tests pass on CI
• [x] Documentation reflects the changes where applicable

[mgrover1]

This looks great to me @dougiesquire ! Merging!