Integrate SDPOR into `model-check/reduction` flag

SDPOR and ODPOR can be used as part of the
DFSExplorer (ODPOR should integrate with some
minor details in the DFS implementation, but that's
for a later PR). This commit replaces the simple
`with_dpor` boolean flag used to control the
reduction with the more precise `ReductionMode`
that is newly extracted from within `DFSExplorer`
and is instead made "public"