Rewriting rules for filter operator

[hartig]

Thanks to the PRs #160 #166 #173 and #175 and commit da994dc supports SPARQL FILTERs natively now. However, the corresponding physical operator (PhysicalOpFilter) is not yet considered explicitly during query optimization. In order to do so we need rewriting rules related to this operator. The purpose of this issue is to define such rules and to implement them.