[ForceAtlas2] Implementation of iteration control (+ config fix)

Hiyo,

I've implemented configuration parameters to control the force atlas iterations. In some graphs, the algorithm is unable to converge fast enough even though it's enough for a human ; hence inducing a lot of node flickering and a massive cpu load for nothing. It is not possible to control this outside the layout manager (like in d3's force layout with the tick() method), that's why I've added two new parameters:
 * ```maxIterations``` is the maximum number of FA2 passes to perform (fixes convergence) ; default none
 * ```initialIterations``` is the number of FA2 passes to perform upon starting the worker (fixes random layout display at the beginning) ; default none

Additionally, I've fixed a bug where the configuration was not given to the worker using Supervisor.start.

WydD