Some performance improvements.

* Instantiate and solve all (P)BESs using an old mCRL2 version for performance reasons.
* After applying quantifier-one-point, pretty print the pbes, and read it using txt2pbes. This is needed to work around the implementation of pbesstategraph not dealing with associativity in guards. Using this work around we are able to (correctly) identify more control flow parameters in, e.g., Hesselink's registers.
* Update documentation according to the changes in the implementation