Bugfix: variables undefined in __exit__() method.

The class variables rnorm, rnorm_0 and rnorm_old were only defined in the __call() method, so if this is not executed they are undefined in the __exit__() method. Fixed this by setting them to 1.0 in the __enter__() method.
Use mesh specific dx.

Replaced global variable dx (which refers to last grid) by class variable dx which is extracted from grid of pressure function space.