Added async and message packing

Added async and message packing for two-way comms.
Also fixed a couple of bugs, typing and added new message classes.

Add MON and ESF classes

Thanks to the work of dgburr these additional predefined messages have been added.
This release also includes a transition from Travis CI to Github actions for the automated tests.

Updated RELPOS and PL messages

0.2.4

Version bump.

Updated message length checking.

Updated to improve robustness of the message length checking as per pull request.

Updates including new pre-defined messages and placeholder methods

Updates including:

• new pre-defined messages as per the pull request.
• added placeholder methods to allow future addition of creating messages for sending to the UBX device
• updates to travis ci builds

Release with parsing of variable length messages

Updated release to support variable length message types.