Partial caching in RDF graph

[arizzi]

Feature description

As discussed in CHEP24, it would be useful to support Caching (e.g.
RDF.Cache) of a limited number of variables with the other variables
being still available in a lazy way.

Example usage

#suppose rdf has columns Jet_pt, Jet_eta
rdf=rdf.Define("Jet_regressedPt", "slowRegression(Jet_pt)")
rdf=rdf.Cache(["Jet_regressedPt"])
rdf=rdf.Define("Jet_discriminator","computeDiscriminator(Jet_regressedPt,
Jet_eta")
rdf=rdf.TH1F("Jet_discriminator")

Alternatives considered

No response

Additional context

No response