Incorporate EGM96 geoid code into azgcorr, eliminating the need for geoid-spheroid separation files (Bill estimates ~3 days effort)

Some (generally high-brightness) level 1 ATM data contains scattered black flecks (can be quite numerous). Most frequent in band 7, but do occur in other bands. Example screenshot (from EUCAARI day 136):

Looking at the raw data with qcdisplay, the flecks aren't present, but there are short horizontal white lines. These seem to be less numerous than the black flecks and may or may not be related. Example screenshot (same flight):

Probably not worth doing anything about, since ATM is dying and due for replacement. First noticed as part of #115

Casi line 3 have alot of chequers and nav problems as in below image