[DF] RDataFrame filters cannot be cast to a common type, TDataFrame filters could

Some advanced functionality was accidentally removed from TDataFrame during its upgrade to RDataFrame, namely the possibility to cast RInterface<TFilter<...>> to RInterface<TFilterBase>.

It turns out that users were relying on that functionality, making the switch to v6.14 problematic.

A general, user-friendly solution (PR 2182, "[DF] Add common base class to all node types") will be available in master as soon as ROOT-9236 (issues with jitting on 32bit archs) is clarified/worked around/solved.

v6.14 might either adopt that solution or re-introduce the cast operators for RInterface that have been removed by commit 693dba9fc8.