support ITime in CMIP mode

[juliasloan25]

The Oceananigans and ClimaSeaIce models don't use ITime. When we have use_itime == true, we set the dt of each model to be a promoted ITime, implicitly assuming each model supports accepts ITime.

For now, we can run with use_itime = false, but we eventually want to support this option. A simple solution can be to convert the ITime to seconds or dates and use that for the ocean/sea ice, but we may also want a more involved solution.

[simone-silvestri]

We can also start supporting ITime in Oceananigans / ClimaSeaIce. We already support Date clocks