This PR makes the following changes:

• fix Bug[bootstraping]: sparse bootstrapping correctness error #505 and more generally the bootstrapping when bootstrapping.Parameters.LogMaxSlots() is not maximal.
• allow for ciphertext packing when N1 != N2 and the ciphertexts are sparsed relative to N1. N1 (resp. N2) refers to the degree of the ring used outside (resp. inside) the bootstrapping.
• add corresponding tests.
• fix Bug/Regression [<package>]:Bootstrap Evaluator ShallowCopy wrong parameter parser. #514
• add some doc + make some methods private in bootstrapping/evaluator.go (to check).

Regarding the last point, the following methods have been made private:

• pack
• unpack
• switchRingDegreeN1ToN2New
• switchRingDegreeN2ToN1New

The idea is that users could want to pack and unpack themselves (using PackAndSwitchN1ToN2 and UnpackAndSwitchN2toN1) to take advantage of the packing, but the lower level functions are not meant to be used outside of this context. Any feedback on that is much appreciated.