Importing from .kernels and .interpolators namespaces

[reint-fischer]

As of #2247 kernels shipped with parcels are no longer available to import using from parcels import AdvectionRK4, but instead using from parcels.kernels import AdvectionRK4. As discussed with @erikvansebille and @VeckoTheGecko, we like this as a feature, making it explicit for the users, and can do the same for interpolators. On the other hand it would add a step to the migration from v3 to v4.

@VeckoTheGecko you also wrote some style guidelines in #2247, but I was unclear whether you had finished these bullets or I am not understanding them correctly:

• When writing package code, we import directly from the file (i.e., prefer from parcels._core.field import Field over from parcels import Field). This is to make clear the import path

• When writing testing code, prefer to import from the parcels namespace for public API (of course, import from

• The parcels and parcels/kernels are the only init.py files we need to currate. We don't need to specify more init.py files

If we agree this should be the syntax for users, I can start writing this into the notebooks I will work on, edit this in any tests, and include it in the migration guide.

[VeckoTheGecko]

If we agree this should be the syntax for users

No, this is only for developers. Users only use the public API. Or am I misunderstanding?

I think it would be good to track those style conventions in dev-notes through

[reint-fischer]

I guess that was my original question: right now from parcels import AdvectionRK4 breaks, with from parcels.kernels import AdvectionRK4 working.

[VeckoTheGecko]

To eliminate all ambiguity: clarifying my original message from #2247

@VeckoTheGecko you also wrote some developper style guidelines in #2247,

This is wrt. the bullet points on when to import from _core and when not to.

Re.

right now from parcels import AdvectionRK4 breaks, with from parcels.kernels import AdvectionRK4 working.

I think we should update all Parcels code such that AdvectionRK4 is imported from parcels.kernels