Add an AuxReader for NumPy arrays

[BFedder]

Is your feature request related to a problem?

Many of the analysis tools MDAnalysis provides return their results in the form of NumPy arrays. Linking these results to the trajectory frames is currently not as simple as it could be, though. One use case I'm picturing is the slicing of trajectories to obtain only those frames that are similar or dissimilar enough from some reference structure as determined by RMSD.

Describe the solution you'd like

I'm hoping to implement an AuxReader for NumPy arrays as part of my GSoC project.

Describe alternatives you've considered

Do nothing.

Additional context

Further reading on my GSoC project can be found here and here