Feature request: `step` kwarg for residence time related functions

[rkingsbury]

Most core methods in MDAnalysis accept the kwargs start, stop, and step to specify the starting and ending frames of the trajectory to analyze, and how many frames to skip (i.e., only analyze every nth frame where n=step).

In mdgo, the residence time functions like get_neighbor_corr accept start and stop but not step. For RTD in particular, one usually wants to start with the first frame and analyze a long time into the simulation, which can take a very long time if every frame is considered. It would be great to be able to pass step to this method so that a preliminary analysis can be done more quickly.

There may be technical reasons why skipping frames is not advised and/or makes the analysis invalid. But unless that's the case, I think this would be a nice feature to add. I'm happy to help but may not have time to work on it for a while, so I thought I'd open this issue as a placeholder.