In order to this PR to work, RFilter and Datasources expose their names.

Before this commit only the root node, the RLoopManager, kept the list of the custom columns defined by the user. This meant that it was not possible to define two columns with the same names in two different branches.

After this commit, each node in the graph has a map with pointers to all the columns defined up to that node. Nodes in different branches may see different custom column. This allows operations like the following to work:

 ROOT::RDataFrame d(3);
   auto branch1 = d.Define("b2", []() { return 1; });
   auto branch2 = d.Define("b2", []() { return 2; });

- Improved overall code quality
- Fixed possible missing initialization of the custom columns. Now the loop manager has again an observer pointer to all the custom columns, only to initialize them.

This reverts commit 783cddc4.

This reverts commit e56b4a96.

There are two problems with this test and cxxmodules. First, it picks up a
header files from $ROOTSYS/include and builds a library. Naturally, we should
build pcm files, however, building module MathCore requires specific -I to
build the needed VecCore and Vc components. This test does not have access
to the include relevant include paths. Secondly, if we want to reuse the
modules from ROOT which makes most sense we can't because we get a hard
error such as: fatal error: malformed or corrupted AST file:
'SourceLocation remap refers to unknown module, cannot find include/pcms/1WYSNQV9VBZK7/stl-2OZGQN92C38MI.pcm

We can fix the first point by moving out all VecCore-related headers
such as Math/Types.h and all of its includers in a separate module. Thus,
the current test will not require the VecCore (as in the textual case).
Alternatively, we can trace the origin of the fatal error and try to remap
the source locations.

The test should reuse the prebuilt modules in the module cache. However,
due to a mismatch between the module-related flags exported by FindROOT.
There is a mismatch between the -D passed by cmake.

This patch make sure that the build arguments match more closely the build
setup of ROOT. Thus we can reuse the already built module files.

[RDF,Tutorial] Add tutorial processing NanoAOD-like file producing dimuon spectrum from Run2011A CMS OpenData

Use pointer to const object rather than const pointer to non-const
object.

Sometimes the serialised methods will fail to load due to missing files.
This adds more output and checks to verify that required files exist to
better pinpoint the problem.

Removes unused spectator expression.
Removes weird weighting and replaces it with an informative comment.