Clean up the subtract flow

[tjgalvin]

In #324 I merged a PR that adds timestep imaging to the subtract and image flow.

This was the output of a busy week, and encompasses a few small tasks. Some additional functionality needs to be merged in fitscube before more work is carried out.

This issue is to highlight that. Notably:

• the fitscube PR needs to merge in a trimming option which we will then use
• a general clean up on the subtract flow.

On the second point it is basically the same code between the scan and chan axis. These should be refactored to eliminate the condinational branching and have everything under a single codepath. We should create the iterables (either the scan selections or channel seletions) and not repeat the same workflow code.

[tjgalvin]

The usage of the -temp-dir option in wsclean imager should also be more directly coupled to the -name argument creation. At the moment should a changing variable be used (e.g. a uuid realised value) then they may be duplicated.

[tjgalvin]

Add a FitsCubeOptions class so I can better control the number concurrent workers, clean up etc.