merge_source_list creates a master list of sources detected from a list
of observations which have been processed to the point of a creating
an emldetect source list. Sources are compared and those within 2"
of each other have the one with the lower flux thrown out. It assumes
that the source detection has been in two bands, 0.4-1.25 keV and
2.0-7.2 keV, which can be produced by cheese-bands in the default mode.