Refactor universe identity check between groups, optional scipy.spars…

…e import

package/MDAnalysis/analysis/contacts.py

@@ -911,7 +911,7 @@ def __init__(self, grA, grB, refA, refB, radius=4.5, lambda_constant=1.8, beta=5
         results: list 
             Fraction of native contacts for each frame
         """
-        assert(grA.universe == grB.universe)
+        if not grA.universe == grB.universe: raise ValueError("grA and grB should come from the same Universe")
         self.u = grA.universe
         self.grA, self.grB = grA, grB
 

package/MDAnalysis/analysis/distances.py

@@ -31,7 +31,6 @@
 __all__ = ['distance_array', 'self_distance_array', 'contact_matrix', 'dist']
 
 import numpy as np
-from scipy import sparse
 
 from MDAnalysis.lib.distances import distance_array, self_distance_array
 from MDAnalysis.lib.c_distances import contact_matrix_no_pbc, contact_matrix_pbc
@@ -55,6 +54,13 @@ def contact_matrix(coord, cutoff=15.0, returntype="numpy", box=None):
     .. versionchanged:: 0.11.0
        Keyword *suppress_progmet* and *progress_meter_freq* were removed.
     '''
+    try:
+        from scipy import sparse
+    except ImportError:
+        print("scipy.sparse cannot be imported")
+        return
+
+
     if returntype == "numpy":
         adj = (distance_array(coord, coord, box=box) < cutoff)
         return adj