Switch from Cython to Pybind11, use latest ROOT patches

[HDembinski]

A major rewrite that replaces Cython as the tool to generate C++ bindings with pybind11.

• This greatly simplifies the implementation and fixes object life-time issues and memory corruption issues that were present in the old code.
• Exceptions raised in Python during the computation of the objective function are now transparently passed through a layer of C++ code back to Python. This replaces the previous ugly workaround to catch these exceptions and raise them again.

Simultaneously, the code makes use of ROOT patches which have been submitted as PRs and are about to be accepted.

• ROOT's Minuit2 now has a rewritten logging system, which allows us to turn on comprehensive debug output at runtime (before it was a compile-time option)
• The new logging system allows us to finally fix issue MINUIT output not shown in Jupyter notebook #295 by passing the logging to Python's print. This means it will show up in Jupyter notebooks instead of the command-line.

Minor fixes:

• A bug in the new "Parameter at limit" warning was found and fixed, the warning was wrongly triggered also when a parameter was close to zero.