Time: Simplification and rationalisation of userTime

First step towards merging userTime into Time so that post-processing tools
operate with the same userTime mode as the solvers.

etc/codeTemplates/BC/BC.C

@@ -34,7 +34,7 @@ License
 template<class Type>
 Foam::scalar Foam::CLASS::t() const
 {
-    return this->db().time().timeOutputValue();
+    return this->db().time().userTime();
 }