Destructors for all classes

[erikvansebille]

One of the major sources for memory leaks in Parcels is the lack of destructors (__del__(self)) functions, which are called when an object is deleted. This accounts mostly for dynamic member variable data (e.g. Numpy/xarray/dask matrices) that are initialised in the constructor, expanded during execution and so on. But, for the removal, Parcels currently relies on the interpreter finding all the references. That is prone to error, and a class that does not use its member array data should release them.

Thanks to @CKehl for bringing this up