Proper OO-Programming inheritance

[erikvansebille]

While some Parcels classes have ok Object-Oriented behaviour (e.g. the Particle class and its associated JITParticles and ScipyParticles, many others don't.

It would be good to improve the OO inheritance for at least

• the ParticleSet class (e.g. 'build', 'add particle', 'remove particle' as methods)
• the Kernel class (with e.g. 'build', 'compile', 'load', 'unload', 'execute' as methods)
• the ParticleFile class
• ...

Some of this is already implemented in #862, for example in the Kernels, where we have a 'BaseKernel' (in kernelbase.py), from which each kind of 'Kernel' (in 'kernel_node.py' and 'kernel_vectorized.py') is derived from.

This will also help fix #618. Thanks to @CKehl for bringing this up

[CKehl]

for ParticleSet class, see #913 and follow-ups - issue resolved, progressively.