Compute parameters for a subset of atoms #54
Description
Is your feature request related to a problem? Please describe.
I am evaluating the Steinhardt parameters for some large MD simulations, and the analysis is taking a long time, I am interested in only the Q6 in the first hydration shell, but with pyscal it seems you have to take the Q6 of the entire system, you can't use just a subset of the system.
Describe the solution you'd like
I would like to be able to select atoms in a system, it would be nice to have a large selection toolbox as in MDAnalysis (https://docs.mdanalysis.org/stable/documentation_pages/selections.html), but even a way to calculate parameters of just a list of indices would do.
Describe alternatives you've considered
Just removing the unneeded atoms won't work, as the Steinhardt parameters depend on neighbors so I will get artifacts, a work around might be to keep more than I need then remove the further ones, but that seems kind of clunky.
The systems I am working with have on the order of a million atoms and I need to average the results over several thousands frames. This would take several days of compute time for each run with my current setup.