Make treatment of distRDF and RDF more uniform

[eguiraud]

• ROOT.RDF.RunGraphs should accept RResultMaps just like it accepts RResultPtrs, so we don't need to filter RResultPtrs out (which is annoying to do in a way that is uniform for RDF and distRDF). Proposed fix at [DF] Make it possible to convert RResultMap to RResultHandle root#13193 .
• ROOT.RDF.[RunGraphs,VariationsFor] (and ideally also SaveGraph) should just work:tm: when the DistRDF equivalents of the RDF types are passed to them. Proposed fix at [DF] Allow calling RunGraphs and VariationsFor on distrdf results root#13248 .

[eguiraud]

the first part of this issue is addressed by #33 .

the second part is blocked by root-project/root#13248 which is blocked by a problem with pythonizing template functions in cppyy.