Removes the deprecated selections of antennas and beamlets from both observation and digital beam forming. This simplifies the code as well as makes concepts like mask use clearer. This MR:
Merges Input Select calculation into the digital beamforming code,
Removes Beamlet Select as the AntennaField & DigitalBeam will not share SDP instances anymore,
Removes Antenna Mask from observation specifications, as that had to abuse the ANT_Mask, which is meant to mask out non-existing hardware,
Moves selected Antenna Set to DigitalBeam, as the selected set only affects beam forming.