Leftover code clean up from #1508

[leewujung]

This is a reminder to take care of leftovers in #1508:

• Merge _retrieve_correct_beam_group_EK60 and _retrieve_correct_beam_group_EK80 into a single function
• Resolve problem with alternative code or when there is only 1 filter_time in the Vendor-specific group in failed test_ek80_BB_complex_multiplex_NaNs_and_non_NaNs for both files:
  https://github.com/leewujung/echopype/blob/62aeaf19100ed39a07ca4e2e4b7c049b0ef54531/echopype/calibrate/api.py#L127-L164
  For the first file "NYOS2105-D20210525-T213648.raw" , instead of producing Sv with 4 channels, it only produces data for 2 channels -- something is not quite right for the stack-dropna operation, need to figure out what
• Various TODOs to consolidate redundant code