0.6 Update

This changes does not change functionality and will not break any
implementation. All changes are internal only based on recieved feedback.

Changes to Collection Class:

1. Updated contains to just use collection.stream.anyMatch
2. Added parameter guards to protect against invalid elements
3. get() will throw an illegal state exception if the collection is
   empty
4. get() asserts object is not null (Which if the collection contains
   elements, it never should be).
5. JavaDoc has been updated to document these changes

Changes to Tests:

1. Added new tests to assert all errors are thrown correctly
2. Added new test to assert get() never returns null in extreme edge
   cases
3. Increased repeated test amounts
4. Updated int comparison to use assertEquals not assertTrue x == y

Enjoy!
ProbabilityCollection.zip