Fixes issue with conflicting transitions

Fixes issues https://github.com/florianv/petrinet/issues/10

I introduced a class to provide special Petrinet test cases 

https://github.com/florianv/petrinet/blob/7ad9c52bfb11a02dee85654d48b8fd984a50aefd/src/Petrinet/Tests/Fixtures/PetrinetProvider.php

The test cases can also be visualized and executed using the Romeo software (http://romeo.rts-software.org/).

https://github.com/florianv/petrinet/tree/7ad9c52bfb11a02dee85654d48b8fd984a50aefd/src/Petrinet/Tests/Fixtures/PetrinetProvider

Ideally we should add more cases like sequence, concurrency etc..