Added Cookbook examples

Added 2 examples to cookbook.

• creating a list of tg::triangles and adding them to a fresh mesh
• extracting edges from an existing mesh and adding it to a vector of segments

question: add images and complementing add/get methods, i.e. add eges and get faces for completeness sake. mostly similar though

note: pls gib developer access, ich kann sonst kein assignee festlegen :)