Shapelet model

[rmjarvis]

We should have a shapelet model.

This will be another LinearModel class, so I'll hold off doing much work on this until @suchyta1 gets farther along on #5 and that design starts taking shape.

One additional design wrinkle to this one is that we'll want some way to decide on the sigma parameter to use for the shapelets. This sets the overall scale of the shapelet model, and it cannot be varies along with the other coefficients. At least not while keeping it a linear model.

Also, I think we will want to use a single sigma value for the whole field, not let each star determine a sigma on its own. Mostly because sigma is degenerate with other coefficients (at first order), so the interpolation will be messed up if these are allowed to change from star to star.

So I think we will want some kind of MetaFit operation that will take all the stars and do something simple first to get sigma, then take a mean value of that across all the stars in the field, then go back and do the full fits on each star using a fixed value of sigma. So there is a bit of interesting API design here that I'm happy to take input on if anyone has ideas.