Allow adding multiple datafiles to dataset simultaneously

[cortadocodes]

Feature request

Use Case

I want to be able to add any number of datafiles to a dataset at once.

Current state

I can only add one using Dataset.add and, if I just set dataset.files to a list of files on an existing dataset, errors potentially ensue (we picked this up downstream in a service using octue).

Proposed Solution

Add a Dataset.add_multiple method or expand the Dataset.add method to take any number of arguments

[thclark]

or fix the problem of assigning files, which is a more intuitive way of doing it (as evidenced by that being what we tried to do ourselves)? If not settable, files probably needs to be _files?