Constraints on SymmSHE

[crockeea]

We discussed adding the constraint that the PT modulus p must be coprime with the odd factors of m to decrypt (and absorbGFactors?) Adding this constraint limits what types can be used for the plaintext modulus, because the constraint only works if p :: Factored. But this restriction is already in place for, e.g., ring tunneling where we have the same constraint.

[crockeea]

Another constraint to consider adding: divisibility of PT moduli on modSwitchPT, or some other stronger constraint.