Pyodc

* String missing values should be `None` not `NaN`
* Refactor oneshot behaviour (`read_odb_oneshot` --> `read_odb(..., single=True)`)
* Raise correct error on `odc` not found
* Split `codb.py` into a full `codc` module
* Fix miscellaneous bugs

* Fixed automatic selection of integral codecs

* Initial version


[6]: https://github.com/ecmwf/pyodc/issues/6
[ODB-534]: https://jira.ecmwf.int/browse/ODB-534
[ODB-533]: https://jira.ecmwf.int/browse/ODB-533
[ODB-530]: https://jira.ecmwf.int/browse/ODB-530
[ODB-525]: https://jira.ecmwf.int/browse/ODB-525
[ODB-524]: https://jira.ecmwf.int/browse/ODB-524
[ODB-523]: https://jira.ecmwf.int/browse/ODB-523