**Changes**
- Updated developer guide to reflect Python 3 development and to not use pip
from system repositories (as it is too old).
**New**
- Added function `remove_link_type_vlan` that allows to remove vlan
virtual links created with `add_link_type_vlan`.
**Fixes**
- Fixed issue in `add_link_type_vlan` that ignored the ports mapping of the
node.
- Fixed issue in `add_link_type_vlan` that registered the port mapping before
creating it, causing potential issue if the creation failed.