Add MWA ITRF interface and several speed improvements

• The ITRF interface (used by Dp3) is implemented by adding a conversion to j2000 RA/DEC, using Casacore. The beamcode converts this later again to HA/DEC, so is a bit slow, but by using the multi freq interface this is mostly mitigated, and this was the easiest to implement.
• Implements the "span" interface so that multiple frequency values can be requested at the same time, saving some recalculation.
• Adds some checks that if the tabulated beam of two frequencies is the same, to calculate it only once.
• Makes more use of the aocommon::MC2x2 class inside the MWA beam code, mainly to not have to copy values when using the span interface.