Set runtime programmatically in namcouple

[nichannah]

The namcouple needs to include the model runtime (usually in seconds) for a run/submit. This needs to be calculated externally by some kind of submit script. This is unnecessary work because the models already calculate this internally.

This issue will allow the runtime in the namcouple to be ignored in favour of a value which is set programatically as an argument to oasis_init_comp().

Along the same lines, a future issue will allow the model / coupling field timesteps to be set programmatically.