added gap node and block improvements

app/node/block/gap/gap.cpp

@@ -0,0 +1,17 @@
+#include "gap.h"
+
+GapBlock::GapBlock()
+{
+}
+
+rational GapBlock::length()
+{
+  return length_;
+}
+
+void GapBlock::set_length(const rational &length)
+{
+  length_ = length;
+
+  RefreshSurrounds();
+}