Raise SrFitError when bond valences are all zero.

Bond valences evaluate to zero for structure models that have bare
atom symbols instead of cation anion specifications.  BVSRestraint
would produce a constant, structure-independent penalty; this
would be hard to track down for non-expert users.

This closes #15.

Author: pavoljuhas

diffpy/srfit/structure/bvsrestraint.py

@@ -89,7 +89,11 @@ def _validate(self):
         p = self.penalty()
         if p is None or p is nan:
             raise SrFitError("Cannot evaluate penalty")
-
+        v = self._calc.value
+        if len(v) > 1 and not v.any():
+            emsg = ("Bond valence sums are all zero.  Check atom symbols in "
+                    "the structure or define custom bond-valence parameters.")
+            raise SrFitError(emsg)
         return
 
     # End of class BVSRestraint