Fixed bug in work calculation

There was a problem when starting with step>STEP0.
I changed a regtest to verify this

regtest/rt8/COLVAR.reference

-#! FIELDS time d1 d2 d3 mr.bias mr.force2
- 0.000000  1.163  2.690  2.130  4.805 28.828
- 0.050000  1.131  2.726  2.151  4.840 29.042
- 0.100000  1.098  2.779  2.164  4.961 29.765
- 0.150000  1.080  2.800  2.157  4.917 29.500
- 0.200000  1.087  2.788  2.133  4.688 28.125
+#! FIELDS time d1 d2 d3 mr.bias mr.force2 mr.d2_work
+ 0.000000  1.163  2.690  2.130  4.805 28.828  0.000
+ 0.050000  1.131  2.726  2.151  4.840 29.042 -0.161
+ 0.100000  1.098  2.779  2.164  4.961 29.765 -0.324
+ 0.150000  1.080  2.800  2.157  4.917 29.500 -0.487
+ 0.200000  1.087  2.788  2.133  4.688 28.125 -0.648

regtest/rt8/plumed.dat

@@ -18,11 +18,18 @@ d1: DISTANCE ATOMS=1,10
 d2: DISTANCE ATOMS=c1,50
 d3: DISTANCE ATOMS=50,c2
 
-MOVINGRESTRAINT ARG=d2 VERSE=B STEP0=0 AT0=0.9 KAPPA0=3.0 STEP1=200 AT1=1.5 STEP2=400 KAPPA2=1.0 LABEL=mr
+MOVINGRESTRAINT ...
+  ARG=d2 VERSE=B
+   STEP0=-100 AT0=0.0 KAPPA0=0.0
+   STEP1=0 AT1=0.9 KAPPA1=3.0
+   STEP2=200 AT2=1.5
+   STEP3=400 KAPPA3=1.0
+   LABEL=mr
+...
 
 PRINT ...
   STRIDE=1
-  ARG=d1,d2,d3,mr.bias,mr.force2
+  ARG=d1,d2,d3,mr.bias,mr.force2,mr.d2_work
   FILE=COLVAR FMT=%6.3f
 ... PRINT
 

src/BiasMovingRestraint.cpp

@@ -201,16 +201,14 @@ void BiasMovingRestraint::calculate(){
     if(verse[i]=="U" && cv<0) continue;
     if(verse[i]=="L" && cv>0) continue;
     assert(verse[i]=="U" || verse[i]=="L" || verse[i]=="B");
-    work[i]+=0.5*(oldf[i]+f[i])*(aa[i]-oldaa[i]);
+    if(oldaa.size()==aa.size() && oldf.size()==f.size()) work[i]+=0.5*(oldf[i]+f[i])*(aa[i]-oldaa[i]);
     getPntrToComponent(getPntrToArgument(i)->getName()+"_work")->set(work[i]); 
     ene+=0.5*k*cv*cv;
     setOutputForce(i,f[i]);
     totf2+=f[i]*f[i];
   };
-  for(unsigned i=0;i<narg;++i){
-    oldf[i]=f[i];
-    oldaa[i]=aa[i];
-  }
+  oldf=f;
+  oldaa=aa;
   getPntrToComponent("bias")->set(ene);
   getPntrToComponent("force2")->set(totf2);
 }