Added const qualifier to CKKSEncoder encode and decode functions

[harichards]

Added const qualifier to the following functions:

• CKKSEncoder::encode
• CKKSEncoder::encode_internal
• CKKSEncoder::decode
• CKKSEncoder::decode_internal

These functions do not actually modify the CKKSEncoder object. Therefore, they should be declared as const.

[ghost]

All CLA requirements met.