Missing check for optimization direction in SparseParametricMdpSimplifier

[volkm]

The following execution results in an assertion violation:

/bin/storm-pars --prism ../resources/examples/testfiles/pmdp/brp16_2.nm --prop "P<=0.01 [F s=5]" --mode feasibility --feasibility:method pla --region "0.1 <= pL <= 0.9, 0.1 <= pK <= 0.9"

The issue is that no optimization direction is specified in the property and accessing the corresponding variable then fails in https://github.com/moves-rwth/storm/blob/master/src/storm-pars/transformer/SparseParametricMdpSimplifier.cpp#L178.

I could add a check for each simplification function in the class but I was wondering if there is a better place where to check it?

[linusheck]

This might depend on https://github.com/moves-rwth/storm/issues/528, but we can also just add a check everywhere for now