refactor gaussian and sightlines.

fix bresenham.
save old make spectra. will be replaced
bunchu of changes I don't want to mention.

new rewrite idea.

lines will have JeansScale pixels. Sampled directly from 5 Mpc
density field with interpolation, plus Lya as gaussians with sigma
= estimated from JeansScale.
make a sensible 1d xi with RSD mocks.