Pyodc

* Improved github/ci integration

* Fixed [6]: pip install breaks codc

* Fixed [ODB-534]: PyPI package is missing CHANGELOG

* Fixed [ODB-533]: Decode data starting with missing values correctly
* Fixed [ODB-530]: Bitfield column inspection returns incomplete data in pure-Python implementation
* Bumped up required `odc` version number to 1.4.0
* Added missing frame properties accessor to `codc` interface
* Fixed [ODB-525]: Setting odc prefix variable (`odc_DIR`) does not work as expected on macOS
* Fixed [ODB-524]: Keys and values in decoded frame properties are switched on older Python version
* Added test flag to skip `codc` tests on demand (`PYODC_SKIP_CODC`)
* Fixed [ODB-523]: Additional properties parameter is omitted in encode_odb() when string is passed as file
* Fixed package setup metadata
* Added documentation

* Correct support for constant codecs
* Decoding by column short name

* Specify `odc` library location with `odc/ODC_DIR`
* Correct `setup.py` dependencies to include pandas
* Support missing ConstantString values encoded from ODB1 using the `odb_migrator`