Tulip 4.8.1 is available

This is essentialy a maintenance release with many bug fixes and improvements in:
- the OpenGL rendering engine,
- the GUI, especially in the Geographic, Histogram, Python Script, Self Organizing Maps, dedicated views,
- the Import CSV wizard,
- several plugins such as the Bertault (OGDF) layout, Fast Multipole Embedder (OGDF) layout, File System import, Link Communities measure, Quotient Clustering algorithm, TLPB file import, Web Site import.