Initial commit of some work to make the finding of sightlines near to particles work better.
Initial commit of some work to make the finding of sightlines near to
particles work better.

The idea here is that we sort the sightlines by the first coordinate,
and then we can binary search them, instead of needing to check the
coordinates of every single one.

sbird/fake_spectra,sbird/fake_spectra,sbird/fake_spectra