Added the support for opaal (model checking of reachability queries
with multicore machines). This is a special version of the
reactant-centered model modified to produce the UPPAAL syntax accepted
by opaal. This model (in VariablesModelOpaal) works exactly like the
"normal" reactant-cetered model (in VariablesModelReactantCetered, also
the parsing of traces has been adapted so that both models are
supported). However, to use opaal you need to explicitly use the .xml
UPPAAL file form the temp directory.