Store and calibrate EC150 echo strength data

[leewujung]

Right now our parser handles data from EC150 ADCP fine, but we still need to determine how to store the data properly and enable calibrating the data.

Should we store the data 1) as a separate Beam_group3, or 2) along with existing Beam_group1 or Beam_group2?

On the calibration side, we need to make sure all the parameters are stored properly, and arguments to compute_Sv are compatible.

Right now we are skipping all datagrams generated by EC150 (#1357) just so that we can just handle data from other channels sitting in the same file.

[leewujung]

@jmjech @spacetimeengineer this is relevant to #995 - could use your inputs also.