Abstraction engine not considered in canHandle

[volkm]

@Vescatur found that using the abstraction engine always prints an error. However, the model-checking query can still be successfully executed. The problem is that canHandle in utility/Engine.cpp is missing the case for this engine.

An example call is the following: ./bin/storm --jani qcomp/benchmarks/mdp/beb/beb.3-4.jani --janiproperty LineSeized --constants N=3 --core:engine abs

[tquatmann]

The exploration engine seems to be affected, as well.