Use datetime64/timedelta64 for progressbar

[erikvansebille]

The current implementation of the tqdm progress bar in v4-dev converts the time to seconds since start-time.

https://github.com/OceanParcels/Parcels/blob/add8158bd4ef6bd0c4055e7677f8f2455e1b1fe2/parcels/particleset.py#L564-L565

https://github.com/OceanParcels/Parcels/blob/add8158bd4ef6bd0c4055e7677f8f2455e1b1fe2/parcels/particleset.py#L587-L588

It would be neat to have the progress displayed as actual time units (so without the conversion)