Analytical trajectory output

[VeckoTheGecko]

As discussed in person, and mentioned in #1844, in v3 there is some coupling in the codebase between FieldSet and ParticleFile in order to support the writing of analytical trajectories. Uncoupling this would allow for greater flexibility and design in the codebase.

We'll remove support for the writing of analytical trajectories "latest locations", and evaluate in v4 how this can be implemented again (or evaluate if it's needed).

[VeckoTheGecko]

Hmmm. It seems like there is some coupling anyway between Kernels and ParticleFiles in Kernel.remove_deleted via fieldset... This would need to be reconsidered - not as simple a fix as I initially thought