Make kernels a required argument to pset.execute()

[VeckoTheGecko]

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.

Originally posted by @andrew-s28 in #2274 (comment)

[andrew-s28]

@VeckoTheGecko isn't this done in #2274? Or did you make this an issue just for tracking purposes (in which case it can probably be closed as completed).

[VeckoTheGecko]

Totally right! I missed that line during review - thanks :)