sub-daily freq for dfmt.download_CMEMS results in empty files

[veenstrajelmer]

This makes sense, since date_str is the same and subsets to 0 hours. Add test to freq that raises error if freq<D. 24h also causes this issue, while 1D works perfectly fine. 1.5D also causes this issue. It happens with all date_str containing more than just y/m/d. We would also get unexpected behaviour with 2M, since only every other month would be downloaded. Maybe it is wise to only accept D/1D/M/1M.

MWE:

import dfm_tools as dfmt
dfmt.download_CMEMS(varkey='uo',
                    longitude_min=142,
                    longitude_max=144,
                    latitude_min=-45,
                    latitude_max=-43,
                    date_min='2023-12-20',
                    date_max='2023-12-21',
                    freq='6h',
                    dir_output='.',
                    file_prefix='CMEMS_',
                    overwrite=True)