Require dt argument in ParticleSet.execute()

[andrew-s28]

Does what it says on the tin: removes the default 1s time step for ParticleSet.execute() and now requires the user specify a dt argument when calling the function.

Addresses #2136 as a part of #2261. Happy hackathon :)

[andrew-s28]

Oops - forgot to run tests prior to PR. Needs some more work to make sure tests don't break.

[andrew-s28]

Okay, this one was a little more involved than it initially looked, but I think I have things working now.

In order to keep allowing dt as a kwarg after the kernel argument (as a lot of existing Parcels code is written, e.g., pset.execute(AdvectionRK4, dt=...)), it's necessary to remove the default from the pyfunc argument, as an argument without a default can't follow an argument with a default.

I personally think that requiring the pyfunc argument as well as the dt argument is actually better than not; I couldn't find a single example in the documentation that didn't have kernels explicitly specified when calling ParticleSet.execute() (so as far as I can tell, this wouldn't break any examples). But this does go beyond the original issue #2136, so feedback welcome.

[erikvansebille]

Looks good; also agree to make pyfunc a required argument too; this is what users would do anyways

Only one question below

[VeckoTheGecko]

Thanks for the contribution @andrew-s28 ! 🚀