Create TransformBoundedField

Jointcal's improved photometric fit is done on the full focal plane, and thus we need its persisted output to be able to convert pixel coordinates on one CCD to the focal plane. Probably the easiest way to implement this is to create an astBoundedField that we can put into a PhotoCalib.

We may eventually want to replace PhotoCalib's use of BoundedField internally with AST, but for consistency with Jim Bosch's meas_mosaic work, keeping the PhotoCalib interface the same is probably best.

For reference, jointcal's photometric output looks something like f(g(x,y))*h(x,y), where (x,y) are in pixel coordinates, g is the pixel to focal plane transformation, f is a Chebyshev polynomial, and h is some chip-level transform (currently a constant).

Tagging this PairCoding, as it might be nice for me to sit with Russell Owen as he works on it, to aid my familiarity with AST.