Add::toMatrix and Add::toMatrixVector yield differently ordered SparseMatrices

[tquatmann]

The matrices differ in the order of the rows within the rowGroups. While this is not a critical bug, such inconsistencies can lead to, e.g., unexpectedly different verification times. This is because Dds are traversed in different orders when InternalAdd::splitIntoGroups is invoked on either just a MatrixDd or a MatrixDd and an additional VectorDd. When (consistently) changing the traversal order to "first the THEN node, then the ELSE node", the GameBasedMdpModelCheckerTest no longer terminates (which could be an issue with mathsat, but has to be investigated further).