Fboemer/eval add inplace

• See #252

• Implements in-place addition by adding EvalAddInPlace for ciphertext-ciphertext addition to the core cryptocontext API, and implementing in in CKKS, BFVrns, BGVrns, and Null schemes.

  • This speeds up Addition by avoiding an unnecessary allocation of and copy to a new ciphertext

• Primitives such as AutomaticLevelReduceInPlace/LevelReduceInternalInPlace in CKKS may be useful in implementing other operations in-place in the future.

• Lib-benchmark shows significant speedup on HE ciphertext-ciphertext addition in-place

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Benchmark                  Time             CPU   Iterations
------------------------------------------------------------
BFVrns_Add              15.5 us         15.4 us       272136
BFVrns_AddInPlace       13.1 us         13.0 us       322618
CKKS_Add                15.6 us         15.5 us       270203
CKKS_AddInPlace         13.1 us         13.1 us       321676
BGVrns_Add              22.2 us         22.2 us       189182
BGVrns_AddInPlace       15.8 us         15.8 us       266131