Switch GNMAnalysis to AnalysisBase.

[PicoCentauri]

Is your feature request related to a problem?

The GNMAnalysis currently uses its own logic for the trajectory analysis. The AnalysisBase class provides the
same functionality and should be used as a parent class.

Describe the solution you'd like

1. Change the parent class of GNMAnalysis to AnalysisBase
   

   mdanalysis/package/MDAnalysis/analysis/gnm.py

   Line 192 in fa07496

   class GNMAnalysis(object):

2. Move the code in the run method that is outside the loop to _prepare and everything that is inside to _single_frame
   

   mdanalysis/package/MDAnalysis/analysis/gnm.py

   Line 309 in fa07496

   def run(self, start=None, stop=None, step=None):

[orbeckst]

@IAlibay please see comments on PR #3244 — if you don't want it in 2.0 please remove.