Write pixel-based model class

[rmjarvis]

@suchyta1 and @gbernstein have started work on the design of this class already.

This will be the first of our LinearModel classes, so we should put some thought into what aspects of this are general enough to be extracted to a middle-tier base class for other linear models.

During the hack day, we though the following would be useful features for arbitrary linear models:

1. Be able to compute the matrix that transforms from model parameters into pixel values
2. Also compute the derivatives of this wrt a centroid shift

Then the LinearModel class could use these matrices to handle the fitting and centroiding (if we are doing that -- we might want to hold nominal centers fixed) in a completely general way that will also apply to e.g. shapelet models.