Calculate GexpPi once and for all during election initialization.

[JohnLCaron]

Its a bit annoying to have the GexpPi calculations (n * n * quorum exps) done each time you fire up a decryptor. Could do them one time and store them in the ElectionInitialized record.

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GexpPi (see eq 13, eq 74 of the EG 2.0 spec)

g^P(i) can be computed from the public commitments Kij as:

GexpPi = Prod_j(Prod_m(Kjm)^i^m, m = 0..quota-1), j - 1..n.

Operation count: n * n * quorum.
This value need only be computed once for each Guardian for all encryptions.