* trx_toolkit/data_msg.py: use uppercase for KNOWN_VERSIONS
* trx_toolkit/data_msg.py: use tuple for DATAMSG.known_versions
* trx_toolkit/data_msg.py: decorate abstract methods as such
* trx_toolkit/data_msg.py: convert comments into docstrings
* trx_toolkit/data_msg.py: do not instantiate abstract class DATAMSG