G parameter moved from PublicKey to ThresholdKey

We don't need G for our PublicKey since it's now always equal N+1. It's required by the `ThresholdKey` to compile, so I moved it there. I will investigate if for a threshold Paillier scheme G is really needed in the pkey.

thresholdkey.go

@@ -14,6 +14,7 @@ type ThresholdKey struct {
 	Threshold                      int
 	V                              *big.Int
 	Vi                             []*big.Int
+	G                              *big.Int
 }
 
 // returns the value of (4*delta**2)** -1  mod n