Allow setting of ParticleClass variable data after initialisation

[VeckoTheGecko]

On the Particle class level, we should support the modifying of variable data (to_write status, dtype, initial).

These modifications should be validated (i.e., have setters) to avoid unsupported input

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I think it was because I think that I think that set_variable_write_status should be removed from ParticleSet, and instead should be defined on the particle itself

my_particle = get_default_particle(np.float64)
my_particle.variables['depth'].to_write = False

I feel it makes more sense, and also defining this before constructing the ParticleSet would prevent a class of bugs.

You're right that this shouldn't have slipped into this PR. I will add back the test and xfail it pending a future PR along the above lines.

Originally posted by @VeckoTheGecko in #2142 (comment)