We've agreed that it should not be the responsibility of the PhotoCalib to include a Jacobian derived from the WCS, because we don't always want to apply that.  Instead, SkyWcs should have the ability (given a pixel bounding box) to return a BoundedField that evaluates to the determinant of the Jacobian of the pixelToSky transform (aka the pixel area in angular units) at every point in that bounding box efficiently.  This probably involves a new BoundedField class (though it's possible there's a way to do this with TransformBoundedField).

A reference implementation can be found in meas_mosaic's calculateJacobian functions, though a different approach to vectorization may be necessary to get good performance from AST.