OSOceanAcoustics/echopype: Patches and enhancements to file conversion

This minor release includes the following changes:

• Fix bug in top level .nc output when combining multiple AZPF .01A files
• Correct time stamp for .raw MRU data to be from the MRU datagram, instead of those from the RAW3 datagrams (although they are identical from the test files we have).
• Remove unused parameter sa_correction from broadband .raw files
• Make sure import statement works on Google colab

• Parse Simrad EK80 config XML correctly for data generated by WBAT and WBT Mini, and those involving the 2-in-1 "combi" transducer
• Parse Simrad .raw files with NME1 datagram, such as files generated by the Simrad EA640 echosounder
• Handle missing or partially valid GPS data in .raw files by padding with NaN
• Handle missing MRU data in .raw files by padding with NaN
• Parse .raw filename with postfix beyond HHMMSS
• Allow export EK80 XML configuration datagram as a separate XML file

To increase maintenance efficiency and code readability we are refactoring the convert and process modules. Some usage of these modules will change in the next major release.