Added test for CONSTANT with multiple values

regtest/basic/rt26/plumed.dat

@@ -13,9 +13,8 @@ RESTRAINT ARG=s.1 AT=5.0 KAPPA=1.0
 
 d5: DISTANCE ATOMS=10,11
 
-a: CONSTANT VALUES=1.25
-b: CONSTANT VALUES=1.33
-j: SORT ARG=a,d5,b
+a: CONSTANT VALUES=1.25,1.33
+j: SORT ARG=a.v_0,d5,a.v_1
 
 RESTRAINT ARG=j.2 SLOPE=1.0 AT=0.0