find_sources: Smarter component selection to account for PSF error

[ajstewart]

Currently components are selected by searching for the nearest beam.

However Epochs 1 and 2 combined integrated fluxes suffer from the error outlined here: https://github.com/askap-vast/vast-project/wiki/Pilot-Survey-Status-&-Data#2-integrated-flux-offsets-at-overlapping-combined-mosaic-regions because of the different PSFs. In short, the overlapping regions can produce flux errors on the order of 2.5 the true flux.

This needs to be accounted for, and where possible, the component selected should be from the most central beam first, to avoid going to the edges where possible.

Possibly even a warning for when Epoch 1 and 2 are used in their present form.