Added a missing check.

This is required to report errors with weights set to zero by mistake.

Related to #247

src/reference/SingleDomainRMSD.cpp

@@ -52,6 +52,9 @@ void SingleDomainRMSD::setReferenceAtoms( const std::vector<Vector>& conf, const
   double wa=0, wd=0;
   for(unsigned i=0; i<conf.size(); ++i) { wa+=align_in[i]; wd+=displace_in[i]; }
 
+  plumed_massert(wa>epsilon,"It looks like weights used for alignment are zero. Check your reference PDB file.");
+  plumed_massert(wd>epsilon,"It looks like weights used for displacement are zero. Check your reference PDB file.");
+
   Vector center;
   for(unsigned i=0; i<conf.size(); ++i) {
     align[i]=align_in[i] / wa; displace[i]=displace_in[i] / wd;