Improved implementation of multi keyword

Also allow a hybrid syntax such as
AT={1 @multi:{5 6 7} 2}

In this case the first and third arguments are the same across replicas,
whereas the second is 5/6/7 on the three replicas respectively

regtest/basic/rt-multi-1/plumed.dat

@@ -7,6 +7,11 @@ f: COMBINE ARG=d,p1.x,p2.y COEFFICIENTS=@multi:{{1,2,3} {0,0,0} {2,4,6}} PERIODI
 
 PRINT ARG=f FILE=colvar
 
+# equivalent syntax:
+#g: COMBINE ARG=d,p1.x,p2.y COEFFICIENTS=@multi:{{1,2,3} {1,0,3} {1,4,3}} PERIODIC=NO
+g: COMBINE ARG=d,p1.x,p2.y COEFFICIENTS={1 @multi:{2 0 4} 3} PERIODIC=NO
+PRINT ARG=g FILE=colvarg
+
 # different arguments in different replicas
 PRINT ARG=@multi:{d,p1.x,p2.x} FILE=multi
 

src/tools/Tools.h

@@ -154,7 +154,15 @@ bool Tools::parseVector(std::vector<std::string>&line,const std::string&key,std:
   std::vector<std::string> words=getWords(s,"\t\n ,");
   for(unsigned i=0;i<words.size();++i){
     T v;
-    if(!convert(words[i],v))return false;
+    std::string s=words[i];
+    const std::string multi("@multi:");
+    if(rep>=0 && startWith(s,multi)){
+      s=s.substr(multi.length(),s.length());
+      std::vector<std::string> words=getWords(s,"\t\n ,");
+      plumed_assert(rep<words.size());
+      s=words[rep];
+    }
+    if(!convert(s,v))return false;
     val.push_back(v);
   }
   return true;