Aspect unit tests

Implemented base unit tests in order to cover recently discovered issue #73 

While writing the tests, I first assumed that empty `Entity` (i.e. no components) is subject to `Aspect.Empty`, but failed test and subsequent research convinced me of the opposite - thus I updated relevant xml docs and added a note for the test to clarify `Aspect.Empty` purpose.

Note that I omitted any "invalid arguments" tests such as null Entity: `Debug.Assert` already describes methods' contracts.