Operations are ready for integration

I've tried to implement a new concept in la4j: operations. Operation is something that takes an LA entity and returns a generic result (`Void` for in-place operations).

There is also iterators introduced with various methods like `orElse` and `andAlso`. So, iterators may be intersected or united in a very neat way.

I believe this concept (operations + iterators) will allow la4j to do the next significant step and solve 2 major issues: an efficient using of sparsity and in-place operations.