fix cice5 restart pointers

[anton-seaice]

Closes #534

For CICE4 with AUSCOM set, restart pointer file contains:

./iced.xxxx

for CICE5, restart pointer file contains

./RESTART/iced.xxxx

[pep8speaks]

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

• In the file payu/models/access.py:

Line 55:80: E501 line too long (80 > 79 characters)
Line 56:51: E203 whitespace before ':'

[coveralls]

coverage: 58.549% (+0.2%) from 58.361%
when pulling 85b6665 on cice_restart_ptr
into 74be036 on master.

[aidanheerdegen]

LGTM and thanks for the informative issue for the context.

[blimlim]

Changes look good to me!

Slightly unrelated question, but would there be any value in splitting off a separate cice4 driver? I feel like we keep having to make changes for cice4 in the cice.py driver, and then have to override them in the cice5.py driver, and other times try to fix something for cice5, and then override it for cice4 in the access.py driver. I often get confused jumping between the three different parts trying to work out what's actually going on, so am interested to hear everyone else's opinions!