Add restart pruning to OM3 driver

[anton-seaice]

Closes #506

This adds support for date based restart pruning to the access-om3 driver.

This uses the payu framework for restart pruning, the only addition is a get_restart_datetime()
function with the a access-om3 class.

[anton-seaice]

Sorry yet another review :-) @dougiesquire and @jo-basevi

[coveralls]

coverage: 59.546%. first build
when pulling 2c56dc9 on 506-om3-restart-pruning
into 2241178 on master.

[jo-basevi]

Changes look good to me! Just one comment really on the return type when there's an unsupported calendar

[jo-basevi]

Thanks @anton-seaice - it looks good to me

[dougiesquire]

LGTM - thanks @anton-seaice! I only flagged the additional space below to prove I did look at this 🙂

[anton-seaice]

Thanks all - there's no more changes, but this needs approving again :-)