BGV: allow encoding N values when using coefficient encoding

[lehugueni]

Fixes #478.

Now, when a plaintext has its flag isBatched=false, one can encode a maximal number of N (set in the parameters) values onto it. This, regardless of the plaintext modulus set in the parameters.

• Makes the change described above
• Modify BGV tests accordingly: test encoding with N values possible with isBatched=false and test other basic functions (add/sub) with unbatched ciphertexts
• Make documentation more explicit about the maximal number of slots depending on the encoding.

[MGeorgie]

ok