graph validation
Merge branch 'master' of https://github.com/andresviikmaa/ut-systems-modelling-hw3

Conflicts:
	code/Pert2Dot.class
invalidation check
Merge branch 'master' of https://github.com/andresviikmaa/ut-systems-modelling-hw3

Conflicts:
	code/Pert2Dot.class
	code/Pert2Dot.java
merge
final