F3 is now an alias for QVector3
So we got rid of an unnecessary vector class of our own.
This resolves #116 (closed) in a different way than originally proposed: Instead of reusing our own BaseVector3D (as for R3 and C3), we make F3 an alias for QVector3. The reason for this is the close integration with other Q classes in ba3d.
At this occasion, one unrelated commit: fix an img link in the doc.