Layout axial symmetry

Hello,

It could be fine to have a layout plugin to perform axial symmetry (horizontal and vertical)
For tow reasons :
- when you do several versions of a map you need have them in the same configuration. From now you have to re spatilize until you obtain the same configuration
- sigma.js exporter haven't the same localization system so there is a horizontal symmetry between the dynamic map (sigma.js) and the static one (.svg) you produce with Gephi

This request is not very important but it could be fine and probably easy to to
Thanks for your work