Use new last() method for List-type classes

- use first() method in some places as well where it helps clarity

- there are a few remaining cases: git grep 'size()-1]'

applications/utilities/postProcessing/lagrangian/particleTracks/particleTracks.C

@@ -103,7 +103,7 @@ int main(int argc, char *argv[])
     {
         startIds[i+1] += startIds[i] + numIds[i];
     }
-    label nParticle = startIds[startIds.size()-1] + numIds[startIds.size()-1];
+    label nParticle = startIds.last() + numIds[startIds.size()-1];