Merge pull request #40 from michalt/dev

More performance improvements
fixed bug concerning delta-generation for constant polynomials
delta-generation was still buggy... now fixed (for linear + constant systems delta=0)
two cfgs from cfg-analyzer benchmark
small script to convert cfg-analyzer examples to our format
fixed a nasty bug in partial_eval (if one of the values is 0 the monomial should be empty as well---otherwise we get assertion failures!)
patch to print stats (thanks to Michal Terepeta--- I included it despite being only debug-output)
three grammars that lead assertion-failures
linear: never insert a zero vector to generators

This should fix the assertion failure during simplification
in sparse_vec.h.