This reverts commit d1aab1956bc56d2d050b59b5ccaf6576f4ff5dc9.

The `RooRealIntegral` class is smart enough to figure out which
variables the function the integrated function doesn't depend on and
trivially integrates them itself by multiplying with the variable
definition range.

However, if the integration range is a subrange of the variable range,
this was not considered. This resulted in wrong results. for integrals
like `pdf.createIntegral(x, "subrange")`, where the pdf doesn't depend
on x. These kind of integrals can occur in the projections that the
RooAddPdf does, so it's important that they work, and fixing this
partially addresses #11486.

This change also fixes a so-far unknown bug in the `RooXYChi2Var`, which
also used these kind of integrals. Without this fix, the `Integrate()`
feature for `chi2FitTo()` was completely broken, which can be seen in
the output of the `rf609` tutorial with any previous ROOT version. The
tutorial looks okay by chance, because the function is dominted by the
quadratic term in `x` that is constant in the fit. But if one makes this
a floating parameter, the problem gets obvious.

Probably that was the reason why the main model parameter was set
constant to begin with, to sweep the bug under the rug. Now, the
tutorials are updated to have the quadratic coefficient floating too.
And also `stressRooFit`, since the reference file has to be updated
anyway.

To demonstrate that things work correctly now, a new unit test was
implemented that does the closure check of the `integrate()` feature of
the `RooXYChi2Var` with a linear fit function.

Disable several tests who are using hist2workspace, which requires xml, when `ROOT_xml_FOUND` is not set

The idea of the RooGenProdProj is that we divide two integral objects
each created with this makeIntgral() function to get the normalized
integral of a product. Therefore, we don't need to normalize the
numerater and denominator integrals themselves. Doing the normalization
would be expensive and it would cancel out anyway. However, if we don't
specify an explicit normalization integral in createIntegral(), the
last-used normalization set might be used to normalize the pdf,
resulting in redundant computations.

For this reason, the normalization set of the integrated pdfs is fixed
to an empty set in this case. Note that in RooFit, a nullptr
normalization set and an empty normalization set is not equivalent. The
former implies taking the last-used normalization set, and the latter
means explicitly no normalization.

This fixes the performance regression reported in #11814, and a new unit
test is implemented to make sure no new numeric integrals pop up in the
reproducer code to that issue.

Unfortunately, this change means that there will be again warnings about
missing `RooAddPdf` normalization sets, but this is not a regression
because these warnings only got fixed in the 6.28 development cycle in
290b4787.

In `RooAbsArg::redirectServers()`, we don't need to do anything if there
are no new servers or if the only new server is this RooAbsArg itself.
And by returning early, we avoid potentially annoying side effects of
the redirectServersHook.

Since the change to Asimov datasets in `testTestStatistics`, there are
warnings that tell you to explicitly set `SumW2Error()` because you are
fitting to weighted data. That's what is done in this commit.

This means the message level threshold can we changed also to not
silence warnings anymore.

Apparently this is ill-formed with C++17 and fails to compile with
Clang version 14.

Co-authored-by: Jakob Blomer <jblomer@cern.ch>

Co-authored-by: Jakob Blomer <jblomer@cern.ch>

- Oftentimes, read callback registration depends on information that is
only available after the field is connected to a page source, e.g. the
on-disk C++ type version.
Therefore, this commit introduces `RFieldBase::RegisterReadCallbacks()`
that is called at the end of `ConnectPageSource()` to register post-read
callbacks as appropriate. The function can be overridden by derived classes.

- Update `RNTupleView` constructor: check if field has callbacks
after the `ConnectPageSource()` call.