Implement Sparse BLS

[johnh2o2]

Kevin Burdge pointed out a paper that came out last year that would be cool for us to incorporate: https://arxiv.org/abs/2103.06193

The basic idea is when you have a small enough number of observations, you can get rid of the binning part of BLS and the gridsearch over q and this is actually a bit more efficient. Basically: at each trial frequency, you sort the observations by phase and then instead of grid searching every transit parameter (phase_of_left_transit_boundary, width_of_transit), you just choose every combination of pairs of observations, and each pair defines both of these parameters.

It saves you from redundantly searching over too-finely grained parameter grids.