Make ciphertext in power basis representation by default

[tlepoint]

Currently, the lib handles plaintext and ciphertexts in NTT representation. Instead, it may be desirable to handle them in PowerBasis representation.

• We should make sure that such a change doesn't impact the performances (it will some of them, so it's important to measure to decide if the change is worth it);
• The expansion, instead of multiplying with a monomial, should use the fact that we are in power basis representation to easily multiply.
• To still allow for fast multiplication by plaintext, it should probably store both the poly and poly_ntt