Marc/fix proof storage

[marcbeunardeau88]

We used the same combiner alpha at every round, asking the storage provider to provide an eval of sum_i alpha^i P_i(r) for a verifier chosen r
to prove knowledge of the P_i.
He could just save one polynomial sum_i alpha^i P_i
We change that to sum_i r^i P_i(r), reusing the r challenge