Remove special handling of single atomgroup modules
Since we removed the SingleGroupAnalysisBase
/MultiGroupAnalysisBase
with the new class design, we should also remove the differential treatment of modules that only support a single atomgroup. The simplest way would be to make the modules take the first atomgroup in the selection and throw a warning about disregarding the rest.
@PicoCentauri and I had the idea that the ability to process multiple atomgroups could be abstracted away from the modules at some point (i.e. the analysis of multiple atomgroups just calls the module multiple times), making the modules themselves easier to maintain.