Test time invariant datasets

[ellesmith88]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR is for the issue Make sure file_namers etc. work for time invariant datasets #111
• [x ] Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• HISTORY.rst has been updated (with summary of main changes)
• bumpversion minor has been called on this branch
• I have added my relevant user information to AUTHORS.md

• What kind of change does this PR introduce?:
  Makes adjustments for files that don't have a time dimension so that they can be processed.
  Works with changes made to file name template in the equivalent roocs-utils branch (time_invariant_datasets)

• Does this PR introduce a breaking change?:

• Other information::