Added unit tests and changed version to 3.5.3

Bug fix in polyarithsmallmod tests

Version 3.5.1

Added unit test for multiply_plain_inplace when plain is a monomial.

Faster validity checks in evaluator for _plain operations' plaintext

inputs

Fix running unit tests in Mac

Update NuGet package metadata to be compliant

Updated Changes.md

Minor edits to documentation.

Minor updates to Changes.md and CKKSENcoder.cs