Override yatm model class method for prior restarts

[jo-basevi]

Implements solution described here: #523 (comment), which was to add an override the parent class get_prior_restart_files in yatm, to return just an empty array. This is to prevent error messaging on a missing prior restart directory when yatm driver does not require any.

Closes #523

[pep8speaks]

Hello @jo-basevi! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file test/models/test_yatm.py:

Line 7:1: E302 expected 2 blank lines, found 1

[coveralls]

coverage: 58.546% (+0.2%) from 58.332%
when pulling 6143912 on ACCESS-NRI:523-yatm-get-prior-retart-files
into 2faf9e7 on payu-org:master.

[aidanheerdegen]

LGTM