Reject invalid frequencies from apt.dat in the loader, and fix
25Khz encoding to be the exact value (i.e round frequencies ending
in 20 and 70 to 25 and 75)
Fix ATCdialog to show 3 digits of comm radio frequency at all times,
and remove its rounding-conversion.
Finally, expand the tests to capture the new behaviour. Test for
EPLL is disabled for now because it contains an invalid frequency.
adae75821d