Time Convention

As found here: https://github.com/OSOceanAcoustics/echopype/issues/656

Time coordinates in Platform group

• time1: will be used for data variables related to GPS lat/lon

  • this will be the same for all sensors

• time2: will be used for data related to platform motion and positions, such as pitch/roll/vertical_offset

  • EK60: source ping_time (not yet implemented)

  • EK80: source mru_time (already implemented)

  • AZFP: source ping_time (added in Rearrange AZFP attributes/variables and remove a variable according to #642 #669: tilt_x/y -- these may eventually be removed)

• time3: will be used for anything from environment type of datagrams, such as water_depth

  • EK60: ping_time (not yet implemented)

  • EK80: environment_time (already implemented)

  • AZFP: X (not yet implemented)