R: added a benchmark for the FR layout.
C: added float type vectors.
Stimulus: added DEPRECATED data type.
Fruchterman-Reingold implementation rewritten from scratch.

It is faster now, and allows layouting on a rectangle.
R/Python interfaces were updated (Python not tested), but
they do not support 3d layouts right now. This will be re-added soon.
Merge branch 'develop' of github.com:igraph/igraph into develop