Add type annotations, remove unnecessary returns, remove unused imports.

[Smarker]

This PR removes most of the warnings on intellij

• Added public type annotations according to the scala style guidelines
  All public methods should have explicit type annotations. Type inference may break encapsulation in these cases, because it depends on internal method and class details. Without an explicit type, a change to the internals of a method or val could alter the public API of the class without warning, potentially breaking client code. Explicit type annotations can also help to improve compile times.

• Removed unused import statements

• Removed unnecessary return keywords

• Edited scala doc comments - changed @result to @return