Declare data dependencies in ThOr functors

By implementing the data_dependencies API added in LHCb!2930 (merged), PyConf can now retrieve data dependencies of functors. The various BoundFunctor specialisations now propagate the dependencies of their inputs to themselves. Complex functors, like MVA and COMB, are responsible for propagating data dependencies from their arguments (which is rather ugly, but I think it's good enough for now).

To support existing tests and allow for gradual migration, strings are allowed in place of DataHandle arguments, but a warning is logged. Users of functors should migrate to using DataHandle.

See Moore#51 (closed) for further motivation.

Requires LHCb!2930 (merged).