Implement message type TDATA

[henrykrumb]

This PR implements the first composite message type, namely TDATA. In the API, the message object is constructed by creating a TDataMessage-type object, supplying an iterable (e.g. list) of TDataRecord objects in the constructor.

The code has been successfully tested with the unit test file, but a test with the OpenIGTLink reference implementation still needs to be done.