FieldSet checking and kernel compatability

[VeckoTheGecko]

In main we have check_fieldsets_in_kernels which defines restrictions on the FieldSet. I think that these can be implemented a different way directly via the kernel API in v4.

I propose that we remove check_fieldsets_in_kernels for now and implement this down the line in v4 - using this issue to track discussion.

Code

https://github.com/OceanParcels/Parcels/blob/c42ca559fb86c10190aee7f2dee199021bada9f4/parcels/kernel.py#L320-L381

[VeckoTheGecko]

Hmm, actually this can't really be removed at the moment since it patches the FieldSet. Let's still keep this open so that we can reconsider how its done in v4