API changes: `particleset.py` and `grid.py`

[VeckoTheGecko]

Contributes to #1695

Changes

• API changes
• Other
  • Updated test_deprecations.py structure (parity with the spreadsheet)
  • Updated pytest conf adding slow and flaky markers

  • skip MPI tests if MPI isn't installed

This PR makes many of the names in particleset.py and grid.py:

• private - in the case where users are not expected to use them (attributes and methods), and
• read only where edits to said variables would provide unexpected behaviour/side effects (attributes)

All privatisations are done in a non-breaking way, raising a deprecation warning.

The method/attributes affected are:

class	name	action
ParticleSet	repeat_starttime	make_private
ParticleSet	repeatlon	make_private
ParticleSet	repeatlat	make_private
ParticleSet	repeatdepth	make_private
ParticleSet	repeatpclass	make_private
ParticleSet	repeatkwargs	make_private
ParticleSet	kernel	make_private
ParticleSet	interaction_kernel	make_private
ParticleSet	repeatpid	make_private
ParticleSet	active_particles_mask()	make_private
ParticleSet	compute_neighbor_tree()	make_private
ParticleSet	neighbors_by_index()	make_private
ParticleSet	neighbors_by_coor()	make_private
ParticleSet	monte_carlo_sample()	make_private
ParticleSet	error_particles	make_private
ParticleSet	num_error_particles	make_private
Grid	xi	remove
Grid	yi	remove
Grid	zi	remove
Grid	ti	make_private
Grid	lon	read_only
Grid	lat	read_only
Grid	time_origin	read_only
Grid	mesh	read_only
Grid	cstruct	make_private
Grid	cell_edge_sizes	read_only
Grid	zonal_periodic	read_only
Grid	zonal_halo	read_only
Grid	meridional_halo	read_only
Grid	lat_flipped	make_private
Grid	defer_load	read_only
Grid	lonlat_minmax	read_only
RectilinearGrid	lonlat_minmax	read_only
Grid	load_chunk	make_private
Grid	cgrid	make_private
Grid	child_ctypes_struct	make_private
Grid	gtype	make_private
Grid	xdim	read_only
Grid	ydim	read_only
Grid	zdim	read_only
Grid	z4d	make_private
Grid	depth	read_only
Grid	check_zonal_periodic()	make_private
Grid	add_Sdepth_periodic_halo()	make_private
Grid	computeTimeChunk()	make_private
Grid	update_status	make_private
Grid	chunk_not_loaded	make_private
Grid	chunk_loading_requested	make_private
Grid	chunk_loaded_touched	make_private
Grid	chunk_deprecated	make_private
Grid	chunk_loaded	make_private
RectilinearGrid	lon	read_only
RectilinearGrid	xdim	read_only
RectilinearGrid	lat	read_only
RectilinearGrid	ydim	read_only
RectilinearGrid	lat_flipped	make_private
RectilinearGrid	zonal_periodic	read_only
RectilinearGrid	zonal_halo	read_only
RectilinearGrid	meridional_halo	read_only
RectilinearZGrid	gtype	make_private
RectilinearZGrid	depth	read_only
RectilinearZGrid	zdim	read_only
RectilinearZGrid	z4d	make_private
RectilinearSGrid	gtype	make_private
RectilinearSGrid	depth	read_only
RectilinearSGrid	zdim	read_only
RectilinearSGrid	z4d	make_private
RectilinearSGrid	xdim	read_only
RectilinearSGrid	ydim	read_only
RectilinearSGrid	lat_flipped	make_private
CurvilinearGrid	lon	read_only
CurvilinearGrid	xdim	read_only
CurvilinearGrid	ydim	read_only
CurvilinearGrid	lat	read_only
CurvilinearGrid	zonal_periodic	read_only
CurvilinearGrid	zonal_halo	read_only
CurvilinearGrid	meridional_halo	read_only
CurvilinearZGrid	gtype	make_private
CurvilinearZGrid	depth	read_only
CurvilinearZGrid	zdim	read_only
CurvilinearZGrid	z4d	make_private
CurvilinearSGrid	gtype	make_private
CurvilinearSGrid	depth	read_only
CurvilinearSGrid	zdim	read_only
CurvilinearSGrid	z4d	make_private
CurvilinearSGrid	xdim	read_only
CurvilinearSGrid	ydim	read_only