Minor changes to the interpreter
- Recursive rule calls led to a NullPointerException - fixed by using a
stack of workcopies
- Added a guard to prevent rulecalls from returning null - return undef
now
- Commented out a call to cleartree that caused significant performance
issues with rulecalls (especially recursive ones; compare fibonacci
sample spec)

Change-Id: Ic9692f8df9fb7de0350a969b1801388da3a51abb
Signed-off-by: mstegmaier <michael-1.stegmaier@uni-ulm.de>