"of_roots is too large" when creating CKKSEncoder

[crockeea]

When using a very large ring (m=262144), I get an error when creating a CKKSEncoder. This is due to https://github.com/microsoft/SEAL/blob/master/native/src/seal/util/croots.h#L38, which sets root_fidelity_ to 131072. I'm not sure what this does, but it has the effect of limiting the size of the ring that can be used.

[WeiDaiWD]

I've changed the generation of roots while addressing the issues that you have addressed previously. The limitation no longer exists. You will see that in the coming release. I'll also try to test the library with 262144.