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).
I don't know if this is a problem for you but in theory you could have mesh nodes on a surface that are not identical with any surface-points. (E.g. a mesh node that is located within a surface-triangle and coplanar with all triangle points).