Code cleanups

[erikvansebille]

This PR combines some code cleanups and minor bug fixes that I stumbled upon while developing #1612

[VeckoTheGecko]

Looks good to me. I don't have any suggested changes. The C code change in codegenerator file is a bit of a mystery to me at the moment, feel free to go ahead if you're confident.

Also, what is the role of the SampleP kernel? Something to to with triggering an update? If the tmp variable name itself isn't important (but the assignment is), could we use _ as a varname instead in line with python convention?

[VeckoTheGecko]

@erikvansebille do you know the role of the assignment in tmp = fieldset.P[particle]?

[erikvansebille]

Yep, it's to trigger the sampling of the P-field, and therefore the assignment of particle.xi on the second (P) grid. Before that, these particle.xi are zero (not sampled yet). See these asserts
https://github.com/OceanParcels/parcels/blob/a8cb61fa8ddc6ae5002538912e757d18a29fc7c4/tests/test_particlefile.py#L322-L323