Modify time attributes for EUMETSAT AMV products

[strandgren]

When loading a FCI AMV NetCDF product witht he fci_l2_nc reader the time attributes represent the following:

• start_time: end_time of the first image/repeat cycle used for the wind tracking
• end_time: end_time of the last image/repeat cycle used for the wind tracking

These are time stored in the FCI L2 AMV filenames.

However, it would make more sense to have the following:

• start_time start_time of first image/repeat cycle
• end_time end_time of last image/repeat cycle
• wind_time end_time of second image/repeat cycle

Then the start_time and end_time would represent the full duration of the data used for the tracking which makes sense.

We also propose to add the attribute wind_time representing the end_time of the second image/repeat cycle.

This logic should then be harmonized among the EUMETAST AMV datasets read with the fci_l2_nc, fci_l2_bufr and seviri_l2_bufr readers.

We should also make sure to add test for the delta between the start_time and end_time to make sure that this is always consistent and breaks if there would be a corresponding change in the L2PF filename convention.

[strandgren]

@YouvaEUMex ^