Mtdata2 compatible

Hi,

Yesterday I sent you a message about the xsens ros driver. I already solved the MTData2 message parse problem. It was just missing ':' (as you can see in commit tab) :)
Is there a reason for that?
`start` is initialized at the start of `read_msg` so that the whole read doesn't take more than `self.timeout`.
Now the outer loop might never stop as you're pushing `start` inside this loop.
I would propose to remove that or fix the potential issue in another way if it's really needed.