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.
Let's keep this structure clean. Computing N^2 is just a really small piece of encryption/decryption work and if we decide we need to tweak performance, we will need a different approach anyway.
Piotr Dyraga