Explainable error message that Field time dimension must be timedelta/datetime

[VeckoTheGecko]

It appears we may need to coerce time to be a dimension and coordinate with knowledge of the parent simulation timestep, if reading NetCDF output straight from MITgcm.

I think we should just not support int time dimensions. If a user wants to use MITgcm runs without a calendar, they have to update the time dimension to timedelta or datetime/cftime on the xarray dataset level before passing to Parcels. We can have an explainable error message denoting the problem.

Originally posted by @VeckoTheGecko in #2060 (comment)

[VeckoTheGecko]

Just realised this is already tested in tests/v4/test_field.py::test_field_init_fail_on_float_time_dim and users get an informative error message

ValueError: Expected right to be a np.timedelta64, datetime, cftime.datetime, or np.datetime64. Got <class 'numpy.int64'>.
E           Error getting time interval for field 'test_field'. Are you sure that the time dimension on the xarray dataset is stored as timedelta, datetime or cftime datetime objects?

[VeckoTheGecko]

Keeping open so we don't forget to mention this in the documentation itself