refactoring entities and added javaDoc
code refactoring: addTopic and addUser now receives User and Topic as
input parameter
add method getLastId in Topic and User Repository
test: add second constructor to BaseEntity for User
Test. Serialization. Incorrect.
Serialization, Deserialization Users.
Serialization/Deserialization Users, Topics, Comments
Serialization/Deserialization Users, Topics, Comments