Added getOutputFromLayer method
Added a test to check if the constructor creates a copy of the input
layer
Added add method test

This test will check if a copy of the input layer exists or not. Also it
will check if the layer was added or not.
Added a test to check addAll method
Added set method test
Updated add method test

Now this test will check if it saves a new copy of the layer when it is
called with an index
Removed unused imports
Added some little explanation to the asserts