This patch provides a fix for ROOT-9344. Support for the ill-defined way to pass
the tree name in the url as '/tree_name' is limited to cases where the substring
'.root' is contained in the file name but not in the tree name.

Adds deprecation messagges for the old practice.

Add now correct Xavier and Xavier normal (glorot) initialisation of weights
Add option to give a seed to weight initialisation (weightInitSeed)

We currently have two checks (that are supposed to have the same
result) in the changed if-stmt and a few lines above where we
set cpp_module_file. Obviously if we generate a module, we should
also set cpp_module_file, however (due to the duplicated code) in
the case of multidicts we generate a module without specifying the
file. This probably causes CMake to behave incorrectly when rebuilding
PCM files.

We generate C++ modules, but we currently don't install them where
they belong.

ROOT needs an overlay files in the include directory during runtime
to correctly mount the system modulemaps for libc/STL. For this
we need to generate and install a new overlay file that points to
the *installed* modulemaps for those libraries.

We can't use the existing modulemap as this one still points to the
original build directory.

We also need to install the related stl/lib modulemaps.

The range update was applied to the wrong leaf and in the wrong direction (it needs to be
applied to the leafcount).

This meant that overlapping basket where registered twice and/or improperly reported as needing vetoing.

by avoiding linear search (caching when needed, passing known information when available)

Fixed use of TString to compile with gcc 8.1

Unloading the enum forward decl has tragic consequences e.g. for template
specializations relying on that decl (as template type parameter): the
enum definition will not be associated with teh forward decl, and any
subsequent template specialization will not recognize the type identity
of the enum forward declaration and its definition.

Instead, silence the diagnostic. This is not nice, but will be fixed by
C++ modules.

This reverts commit 42f889c0.

Remove check in TFormula if functions are valid by looking at TClass or global functions in gROOT. Leave the check to Cling