martypy should accommodate ROSSERIAL messages of differing lengths to allow for message extension

[robdobsn]

martypy does not handle variable length ROSSERIAL messages. Hence it is not currently possible to accommodate additional fields in these messages - which are added to the end of the message - without martypy breaking. martypy should be updated to accommodate variable length ROSSERIAL messages.

For instance this code - currently at line 162 in RICROSSerial.py:

def extractRobotStatus(cls, buf: bytes) -> Dict:
        robotStat = struct.unpack(">BB", buf)

This fails if the size of buf is not exactly 2 bytes. It should be changed to something like (where cls.ROS_ROBOT_STATUS_BYTES is the number of bytes in the default size message)

    if len(buf) >= cls.ROS_ROBOT_STATUS_BYTES:
        robotStat = struct.unpack(">BBIIIIIBB", buf[0:cls.ROS_ROBOT_STATUS_BYTES])