Simplify the PotentialSource build functions.

Small refactor.
Right now, to create a PotentialSource object, we have to do
```python
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
```python
# call build directy
ps1 = koffi.PotentialSource()
ps1.build(pixel_positions, mjds, format = "xy")

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

Start by inspecting PotentialSource, its constructor, and build_from_images_and_xy_positions to understand the current construction paths. Add the requested build and constructor call forms while preserving the existing behavior, then verify that both examples create equivalent PotentialSource objects.

Written by the indexing model from the issue text.