Combine multiple EchoData objects

[leewujung]

Right now we have open_raw that handles parsing raw data from a single file into a nice EchoData object. Let's revamp the previous combine=True option so that we can combine multiple EchoData objects explicitly. This function can then be used under the hood for when we put together an open_mfraw function.

Notes from #180 for Environment group:

• For EK60: these parameters are encoded with each ping (each sample datagram)
• For EK80: these parameters are encoded with a single ping_time before all sample datagrams
• For AZFP: temperature and pressure of the instrument is recorded with each ping, but not salinity, which has to be supplied by the user during calibration.

[imranmaj]

Implemented in #383, #414, #422, #425