Rename `pyfunc` argument in Kernels and ParticleSet to `kernels` or something similar?

[erikvansebille]

The name pyfunc argument in kernel.py is a remnant from when we converted python functions to C code in v3. Now that we don't do that anymore in v4, the name may not be very informative/descriptive

Since the pyfunc expects a function with the signature of a Kernel or a list of such functions (or a Kernel object), I'd suggest we rename it to kernels. Or does anyone have a better name for it?

[VeckoTheGecko]

I think kernels is good for the pset argument.

For the Kernels object itself, I think let's leave that as it - having a kernels argument there is quite circular in the vocabulary that we're using. Also I'm not really sure if we even need the Kernel in the public API of parcels, or even in the internals - looking through now most of the code is to do with v3 code and should probably be reconsidered. A topic for another issue.