merge SAT verifier

[Songtuan-Lin]

Hi Gregor & Daniel,

This is the request for merging the SAT verifier.

The verifier is a subproject being placed inside the folder verifier.

However, I made some small changes to the code and the CMakefile for the SAT planner, because otherwise, it can cause some error when building the verifier.

You could check whether you are satisfied with those changes, and if you don't want to commit those changes (to the SAT planner), I could figure out other way to resolve the compiliation errors :)

Thank you so much!

Cheers, Songtuan