Implementing View for KernelParticle/ParticleSet

[erikvansebille]

This PR fixes #2143 by creating a ParticleSetView class that links a View of the ParticleSet (on the kernel level) to a ParticleSet.

This allows much more intuitive masking in Kernels, see e.g. the change in the Argo and the interaction tutorials.

For example, some of the old code for the Merge Kernel in the interaction tutorial was

masses = particles.mass
states = particles.state

masses[larger_idx] += particles.mass[smaller_idx]
states[smaller_idx] = parcels.StatusCode.Delete

particles.mass = masses
particles.state = states

While the new code is

particles.mass[larger_idx] += particles.mass[smaller_idx]
particles.state[smaller_idx] = parcels.StatusCode.Delete

Note that this PR is blocked by #2444 (fix cherry-picked into this PR as 9aa0f32)

[erikvansebille]

Is this particle.py file the best place for the ParticleSetView and ParticleSetViewArray classes? Or better to move them to particle.py; or its own file?