Restrictions on MITgcm when changing timestep

There are a number of existing issues to do with supporting changing the model timestep in MITgcm

The changes above use an approach which places restrictions on what can deltaT can be changed to:

has to be a divisor of the current model time
newly calculated nIter cannot be the same as nIter with the previous time step

To overcome these issues it should be possible to change the model baseTime and reset nIter.

payu-org / payu