Echopype

This release contains updates to the convert and model module:
- Correct file parsing and storage for split-beam angle.
- Allow user-defined environmental variables to be used in calibration.
- Accommodate EK60 setting variabilities in the `range_bin` dimension.
- The raw file will be split into multiple .nc files if the length of `range_bin` changes in the middle.
- For cases when different frequency has different length of `range_bin`, the shorter channels are padded with `NaN`.
- Allow combining multiple files during unpacking.
- Options to:
- overwrite previously converted files.
- specify output directory and filename postfix for `.calibration()` and `.get_MVBS()` operations.
- Add support to convert to .zarr file. Note processing functionalities have not been fully supported.

This release includes major changes to the package structure and functionality improvements:
- Added support for handling `.01A` data files from the AZFP echosounder.
- Changed the module structure to enable a uniform interface to work with EK60 and AZFP data.
- Improvements to EK60 components:
- Included NMEA datagram unpacking in file conversion routine.
- Added handling for pings with different range setting within the same data file. This feature is experimental and the default behavior will change in future release.

Minor update:
- use versioneer for auto-versioning