add isAcyclic function in the GraphProperty class

[Gillu13]

I added an "isAcyclic" function in the GraphProperty class + some unit tests. Hope this was the right place to add this.

[Gillu13]

OK, I need to change the array declaration to be complient with php 5.3

[clue]

@Gillu13 Thank you for filing this PR 👍 It looks like this might be better off in a separate class (see also #33) and this should probably be tested against more graphs (i.e. empty, edgeless, loop edges etc.), is this something you're planning to look into?

I'd first like to get a maintenance release out to bring this library up to date before introducing any additional algorithms. Once this is done, I'm looking forward to merging more of these algorithms, so I hope you're still interested in getting this merged then

Again thank you for your valuable contribution! I understand this long wait time can be frustrating, so rest assured I'll try to get these intermediary release out as soon as time permits (this project doesn't currently get any sponsoring, so this happens mostly in my spare free time) and will then look into this PR again in more depth! 👍

[Gillu13]

Sounds good! cheers