static PageRank allow checkPoint from previous computations

It would be really helpful to have the possibility, when computing staticPageRank to use a previous computation as a checkpoint to continue the iterations.

I have done a small code proposal, but there is a problem because at the end of the iterations a normalization step is performed.

Therefore, if we use this normalized graph as a checkpoint for a new set of iterations, the algorithm will not restart in a coherent way.

I created a branch in my fork: https://github.com/JoanFM/spark/tree/pageRank_checkPoint

I hope you can consider this feature, and if you do that you can implement yourself or propose me a way to handle this identified problem

Thank you very much,

Best regards,

Joan