Emit warnings for empty Ag and user grid in DensityAnalysis

package/CHANGELOG

@@ -21,6 +21,7 @@ The rules for this file:
   * 2.0.0
 
 Fixes
+  * UserWarning displayed when empty atomgroup is given to DensityAnalysis (Issue #3055)
   * atommethods (_get_prev/next_residues_by_resid) returns empty residue group 
     when given empty residue group (Issue #3089)
   * MOL2 files without bonds can now be read and written (Issue #3057)

package/MDAnalysis/analysis/density.py

@@ -384,6 +384,11 @@ def __init__(self, atomgroup, delta=1.0,
         self._zdim = zdim
 
     def _prepare(self):
+        if len(self._atomgroup)==0:
+            msg = ("No atoms in selection over whole trajectory")
+            warnings.warn(msg)
+            logger.warning(msg)
+            return
         coord = self._atomgroup.positions
         if self._gridcenter is not None:
             # Issue 2372: padding is ignored, defaults to 2.0 therefore warn
@@ -424,6 +429,11 @@ def _prepare(self):
         self.density = None
 
     def _single_frame(self):
+        if len(self._atomgroup)==0:
+            msg = ("No atoms in selection over whole trajectory")
+            warnings.warn(msg)
+            logger.warning(msg)
+            return    
         h, _ = np.histogramdd(self._atomgroup.positions,
                               bins=self._bins, range=self._arange,
                               normed=False)
@@ -434,6 +444,11 @@ def _single_frame(self):
         self._grid += h
 
     def _conclude(self):
+        if len(self._atomgroup)==0:
+            msg = ("No atoms in selection over whole trajectory")
+            warnings.warn(msg)
+            logger.warning(msg)
+            return
         # average:
         self._grid /= float(self.n_frames)
         density = Density(grid=self._grid, edges=self._edges,