Boundary element generator

This changeset aims to provide a tool to extract boundary elements from an existing domain mesh. The boundary elements will be used when we applying 2nd and 3rd boundary condtions (i.e. we need to integrate shape functions over boundaries).
okay... perhaps i should make a separate issue for this discussion. i'll not touch it in this PR. I did the change only for making my test easier, because created edge elements always follow the same rule for node orientation, i.e. counter clockwise, for every edges.