Infer antenna identity from its name in the ANTENNA table

[landmanbester]

There is currently a check that the number of antennas deduced from the ANTENNA1 and ANTENNA2 columns must be consistent with the number of antennas in the ANTENNA table. This is was a hack to guard against applying incorrect gains during imaging which has turned out to be overly strict in some cases (e.g. when imaging corrected data in which fully flagged antennas have been discarded in the MAIN table). I will simply make this check optional for now (--check-ants parameter). However, since the problem actually stems from using indices instead of antenna names to identify antennas, we should really be doing this in a more consistent way