Make use of or remove the DatasetFactory class

As it stands the `DatasetFactory` class isn't actually used. I assume it was implemented with the intention to replace the logic here.

https://github.com/MetOffice/CDDS/blob/d04ff181c65ada49312bdeddaeeba615e1ce0476/cdds/cdds/qc/command_line.py#L155-L164

So we should either make use of it or remove it.

Inspect the DatasetFactory class and the logic in cdds/cdds/qc/command_line.py at lines 155-164. Determine whether the existing command-line path should use DatasetFactory or whether the unused class should be removed, then verify that the QC command-line behavior remains intact.

Written by the indexing model from the issue text.