Extract Model Checking from CREST into own package

[stklik]

I think it would be better in the long run to extract the TCTL + timed Kripke part from CREST and provide it as its own package.

This would make it reusable and extendable.
Who knows, maybe other people want to join it, too?

[stklik]

Kickstarting the project here:

https://github.com/stklik/pyTCTL