Loading new frame doesn't mark old frame data as stale #2309
Description
Originally from here
https://groups.google.com/forum/#!topic/mdnalysis-discussion/ppcDufYzf9A
Loading a new frame without velocities doesn't flag the old velocities data as stale. I think the TRR reader has logic which set has_velocities = False which makes ag.velocities then raise a NoDataError. This should work universally across all readers.