Add the ability to select two stations to fix the image centroid

[ptiede]

This is experimental and add the ability to select two stations to specify the image centroid. Essentially this locks the gain phase for a single time for two stations. This is mathematically equivalent to forcing the image centroid to specific location.

To finalize this I'd like

1. Automate site selection (longest strongest perependicular baselines)
2. Pick the phase so that the image centroid is near the origin