Singleton generator

[KristianKarl]

Lets a user to reproduce the same path over and over again.

When graphwalker uses the random generator, the seed is logged (on INFO level). For example:

java -jar graphwalker-cli/target/graphwalker-cli-4.1.0-SNAPSHOT.jar -d INFO offline -g graphwalker-io/src/test/resources/json/petClinic.json|head
{"currentElementName":"e_StartBrowser"}
09:47:43.252 [main] INFO org.graphwalker.core.generator.SingletonRandomGenerator - Seed: 67300799054828
{"currentElementName":"v_HomePage"}
{"currentElementName":"e_Veterinarians"}
{"currentElementName":"v_Veterinarians"}
{"currentElementName":"v_Veterinarians"}
:

That seed can be reused when put in the json file, add for instance "seed": 67300799054828, to the file.
See petClinicWithSeed.json for an example.

[codecov-io]

Codecov Report

Merging #218 into master will decrease coverage by 0.1%.
The diff coverage is 68%.

@@             Coverage Diff              @@
##             master     #218      +/-   ##
============================================
- Coverage     76.17%   76.07%   -0.11%     
- Complexity     1176     1187      +11     
============================================
  Files           126      127       +1     
  Lines          4332     4368      +36     
  Branches        655      656       +1     
============================================
+ Hits           3300     3323      +23     
- Misses          802      814      +12     
- Partials        230      231       +1

[KristianKarl]

One problem with this PR is the introduction of a singleton, which complicates things when running unit tests. See https://dzone.com/articles/singleton-anti-pattern
But in this case, it adds a very useful feature. So in this case I'd say it's worth the complications.

[KristianKarl]

@nilols a review pretty plz?