convert selection string to atomgroup object

Here I convert hbond code from selection strings to atom group object issue MDAnalysis#3933.If you think my approach is correct then please let me know.I will update this full code.Please merge this pull request so i can have pull request for my gsoc

package/MDAnalysis/analysis/hydrogenbonds/hbond_analysis.py

@@ -116,8 +116,11 @@
   u = MDAnalysis.Universe(psf, trajectory)
 
   hbonds = HBA(universe=u)
-  hbonds.hydrogens_sel = hbonds.guess_hydrogens("protein")
-  hbonds.acceptors_sel = hbonds.guess_acceptors("protein")
+  hydrogens = u.select_atoms("protein")
+  acceptors = u.select_atoms("protein")
+  hbonds = HBA(universe=u)
+  hbonds.hydrogens_sel = hydrogens
+  hbonds.acceptors_sel = acceptors
   hbonds.run()
 
 Slightly more complex selection strings are also possible. For example, to find hydrogen bonds involving a protein and