Make TPRParser also read coordinates

[richardjgowers]

So a TPR file also has coordinate and velocity information inside it. We should be able to make a Reader for this format. This would then let u = mda.Universe('myfile.tpr') yield position information as well.

Because of how Universes are created, the Reader would be completely separate from the TPRParser (but could reuse components) and would have to subclass SingleFrameReader. Basic example of this:

https://github.com/MDAnalysis/mdanalysis/blob/develop/package/MDAnalysis/coordinates/INPCRD.py

[orbeckst]

That would be nice.