Simplify the PotentialSource build functions.

[maxwest-uw]

Small refactor.
Right now, to create a PotentialSource object, we have to do

ps = koffi.PotentialSource()
ps.build_from_images_and_xy_positions(pixel_positions, mjds)

it would be much nicer if we could do it like this

# call build directy
ps1 = koffi.PotentialSource()
ps1.build(pixel_positions, mjds, format = "xy")

# build from constructor
ps2 = koffi.PotentialSource(pixel_positions, mjds, format="xy")