only test_io passes right now.

acc errors seem a bit off though

1 particle species seems to be correctly integrated at the moment.

all traces of temporal interpolation removed.

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".