remove full sample_vec_field from particles

I'm not sure this is the proper way to do things, but I need tox to
work, so I don't care anymore.

kM is used in various computations, kMspec is the one relevant for
spectra.
And they are different for the smooth filter case...

I'm not using the ideal estimate of the integral length since I'm doing
the integral with the 1D spectrum, but I think this should be close
enough.

in practice nothing changes, but the spectra will be shorter and the kM
eta calculation will be "correct".

it's now based on the last git commit