L2SS-1313: Simplify digital beam former

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.