Integrating https://gitlab.cern.ch/will/xroofit by Will Buttinger to get the RooBrowser.

Hidden behind the ROOT 7 flag until C++17 becomes the default minimum standard.

At the moment, it's not really possible to implement a power law or a
exponential of a polynomial (as used, for example, in the ATLAS H->yy
analysis) without using RooGenericPdf. This PR adds two new Pdf classes
that implement an exponential polynomial and a power law.

Canvas names were changed, there is more simple syntax to get sub-pads

* There were several warnings in filter.cxx

* clang format

* more clang format

* another clang format

- Remove the `.so` suffix and any trailing whitespace in the output generated
by `libs.C`.

- Remove dead code in `libs.C`

`modifyClassWebpages.sh` might spawn many `modifyClassWebpage.sh` processes
via `xargs -P`. In turn, `modifyClassWebpage.sh` depends on `libs.C`
which is ACLiC'ed if to generate `libs_C.so` where required.

However, if `libs_C.so` does not exist, multiple processes might race to
create it. This commit ensures that the macro is ACLiC'ed before
`modifyClassWebpage.sh` runs.

- fix `mktemp` command lines to only use template 'X's at the end

- use `sysctl -n hw.ncpu` instead to get the number of available CPUs on Darwin

- move OS-dependent variable assignments to the Makefile

- Doxygen was creating one svg for every class, showing in which library it resides. This created
  6000 svgs, which were mostly identical. Now, only ~100 are created for every library in ROOTSYS/lib/,
  and the relevant webpages link to those.
  This speeds up the "listlibs" step of the doxygen process signficantly.
- Add parallel processing for collaboration diagrams of class webpages.
- Remove scanning of rootmap files. It doesn't work, any more. TCling needs
  a fix for ROOT-10900, so gInterpreter->GetClassSharedLib() will always
  return the right thing.
- Further, parallelise the modification of class webpages.

[ROOT-10157]
- The doxygen filter step now only writes out that it wants a
notebook, and how it should be created, but it doesn't run the notebook.
- Later, this info is picked up by makeNotebooks.sh, and notebooks are
generated in parallel.
- Add a variable in the Makefile to choose number of processes for notebook
  conversion.

converttonotebook.py was converting each comment into a markdown cell.
If comments are inside blocks, the blocks get split into multiple cells,
and therefore cannot execute, any more.
[ROOT-10877]

If errors/exceptions occur during conversion of tutorial macros to
notebooks, a few more helpful messages are printed. Further, the exit
status of the notebook conversion step now propagates to the outside,
the conversion failures can be detected automatically.

- Add a function that measures the indentation of a function body.
- Use this to create nicely formatted cells in notebooks. This is
especially important for RooFit tutorials.

converttonotebook.py was able to recognise a few functions based on a
few hard-coded return types in C++. Now, it searches for patterns such as
   <return type>[*&] [abcd::]abcd(
to find functions, and then it counts () as well as { } to find the
function bodies. It also more reliably deals with things that appear
before function definitions, such as
   template<typename T>

[ROOT-10861] This is a partial fix for ROOT-10861, because functions are
not ripped apart, so the notebook will run.

Further, doxygen-style latex tags such as `\f[` by
notebook-style `$$`, so markdown cells look better.

Make sure converttonotebook recognises indented comments.

* [DF][NFC] Mention change in behavior of Alias in rel notes

* [skip-ci] make sur the http doc is read by doxygen.

Co-authored-by: Enrico Guiraud <enrico.guiraud@cern.ch>

Copy from JSROOT repository

* Fix DF examples to make them work as notebooks

* Add missing space

* python exec in filter.cxx

* use "authors"

* notebook does not make sense for these tutorials.

Since JSROOT v6 all functions names are "onWebsocketMsg"

* precompile

* add missing headers

* MetaSema: add short version of help, as in quit
Primer: join help and ?, mention .quit and .exit
TRint: mention .?
TApplication: mention .? and CINT 2 Cling

* clarify class name with ROOT7

https://docs.mathjax.org/en/latest/upgrading/whats-new-3.0.html#improved-speed

Also the size of the tar file is smaller