Modify the Readme for Robust ECDSA and Implement such modification

[SimonRastikian]

Currently Robust ECDSA documents and implements the scheme while all parties are symmetric in the online phase.
It is possible to make them non-symmetric by having a single coordinator that outputs a signature.

This would reduce the load on the networking layer from O(n^2) to O(n) in the online phase.
Similarly it is possible to send an already linearized elements to the receiver instead of requiring the latter to linearize the shares. This would amortize the reduces some computation load on the receiver in the order of n-1 scalar multiplication and n-1 lagrange coefficients computation.

[gilcu3]

Closed by #92, #93 and #109