Store MDAnalysis XTCReader index for XTC files.

[dotsdl]

To allow quick random access to an XTC trajectory, MDAnalysis generates an index of its frames when prompted. This index can take several minutes to build, and if the Universe in questions is defined with multiple XTC files, it can take far longer. It would be incredibly useful if these indexes could be saved and recalled later.

The XTCReader in MDAnalysis can write and read these indices to and from disk, so the major question is how to implement such that a stale index is not applied to a trajectory that has changed. This will be where most of the work will come.

Addendum: File locking will need to apply here. The existing machinery for doing this (Core.Files) should be used somehow.

[dotsdl]

This may be addressed in the MDAnalysis codebase itself: https://code.google.com/p/mdanalysis/issues/detail?id=208

[dotsdl]

XTC and TRR files now have persistent offsets generated automatically by MDAnalysis.