Remove `print_function`, `absolute_import` and `division` imports.

Remove remaining Python2 backwards-compatibilty code.

- move Gaudi::Funtional to GaudiFunctional subdir
  - headers are now under "Gaudi/Functional"
  - legacy GaudiAlg corresponding header redirect to the new ones
- replace GaudiAlgorithm with Algorithm wherever possible in tests and
  utilities
- add Gaudi::Sequencer basic implementation to replace GaudiSequencer
- move GaudiAlg specific tests, examples, dictionaries and Python modules
  to GaudiAlg subdir
- add CMake option GAUDI_ENABLE_GAUDIALG to allow skipping GaudiAlg
  (default to true)
- minor fixes and clean ups

If `QMTEST_DISABLE_FIXTURES_REQUIRED` is truthy, a failing prerequisite
test will not cause the test depending on it to be skipped.

Workaround for #216

gaudi_add_pytest is used to declare pytest tests. Each of the test
files discovered by pytest are mapped to one ctest test, to be able to
run them in parallel.

Tests are collected when ctest is invoked and/or at build time, and only
in case changes are detected in scanned files.

Helper scripts to prepare the environment for execution to be consistent with
what the toolchain provided (Marco)

Change explicit mention of Python binary to python3
which should be supported by all distributions and
environments (https://peps.python.org/pep-0394/).

Update hash bangs to python3.

Adapt CMake to search for pytest also as pytest-3; likewise
for nosetest binary as nosetests3 (with fallback to
pytest and nosetests, which can be used in some
virtual environments, even with a Python 3 install).

Remove unnecessary utf-8 encoding lines.

Update test scripts to python3

Create a symlink to the built Python binary modules so that they can be
imported as `Package.Module` (same as it is the case once they are
installed).

This is a follow up of !1354. There PkgConfig is only looked up if
the `pkg_check_modules` macro hasn't been define. The problem is that
the scope of macros seem to be global, and finding PkgConfig in another
directory defines that macro. However, variable scope is per directory
and the cmake of PkgConfig doesn't really work.

This changes the check to use the `PKG_CONFIG_FOUND` variable

so that things work in a super project build

This allows for super project builds to have separate merge
targets for every project.