and adapt test.

This reverts commit 5993be60, reversing
changes made to f6b75592.

* Revert "[cmake] Narrow the list of included directories."

This reverts commit 88c16588.

* Revert "[cxxmodules] Build cling runtime into module."

This reverts commit 76a9ce75.

therewith for tens of widely used classes such as variables, constants,
distributions and combinations thereof.

this was due to an improper assumption in the way in which the binning
was accessed.

therewith for tens of widely used classes such as variables, constants,
distributions and combinations thereof.

XrdSecEntity::Reset() is only available in newer versions of XRootD.

ROOT-9595 #close

but add a comment to the jitted code in order to obtain a clear error message if
the user tries to use this type in a jitted action/transformation later in the chain.

There are a number of trivial operations that users often want to
perform on dataframes that are surprisingly hard to get right, for
example adding several Defines in a loop or conditionally adding a
Filter depending on a runtime boolean (both use-cases are challenging in
C++, trivial in python).

Difficulties boil down to the fact that different dataframe nodes have
different types (because their types incorporate e.g. the type of the
callable passed to a Filter and the type of their parent node in the
computation graph).

This commit adds a common base class ROOT::RDF::RNode to all
nodes of the graph (except leaves a.k.a results, which have a completely
different interface), so that users can, for example:

* take any dataframe node by reference in non-template functions as RNode&
* keep dataframe nodes in `std::vector<RInterface<RNode>>`
* keep non-const pointers to dataframe nodes

and so on.

Internal RDataFrame code is also simplified by the introduction of this
common base class.
The downside is that if this mechanism is abused users might end up with extra,
unnecessary virtual calls in their event loop -- on the other hand, this
mechanism should only be used in situations that before required either complex
template magic or dirty and slow tricks.