Add IDYNTREE_DETECT_ACTIVE_PYTHON_SITEPACKAGES CMake option

[traversaro]

This option (OFF by default) can be used to automatically install the Python bindings in the current active site location.

This is convenient for example on system in which the site location is not part of the same prefix in which C++ libraries are installed, for example in Conda in Windows where: CMAKE_INSTALL_PREFIX is %CONDA_PREFIX%/Library and Python3_SITELIB is %CONDA_PREFIX%/Lib/site-packages .

With this option, it is now possible to compile iDynTree on Conda on Windows in a way such that both C++ libraries and Python packages are automatically found:

conda create -n idynpy -c conda-forge cmake compilers pkg-config irrlicht assimp libxml2 ipopt swig python numpy
conda activate idynpy 
git clone https://github.com/robotology/idyntree
cd idyntree
md build
cd build
cmake -DCMAKE_INSTALL_PREFIX:PATH=%CONDA_PREFIX%\Library -DIDYNTREE_DETECT_ACTIVE_PYTHON_SITEPACKAGES:BOOL=ON -DIDYNTREE_USES_PYTHON:BOOL=ON -DCMAKE_BUILD_TYPE=Release ..
cmake --build . --config Release
cmake --install . --config Release

The same script on Linux/macOS (the only difference is that DCMAKE_INSTALL_PREFIX is CONDA_PREFIX instead of <CONDA_PREFIX>\Library :

conda create -n idynpy -c conda-forge cmake compilers pkg-config irrlicht assimp libxml2 ipopt swig python numpy
conda activate idynpy 
git clone https://github.com/robotology/idyntree
cd idyntree
md build
cd build
cmake -DCMAKE_INSTALL_PREFIX:PATH=$CONDA_PREFIX -DIDYNTREE_DETECT_ACTIVE_PYTHON_SITEPACKAGES:BOOL=ON -DIDYNTREE_USES_PYTHON:BOOL=ON -DCMAKE_BUILD_TYPE=Release ..
cmake --build . --config Release
cmake --install . --config Release

I also cleanup a bit the Python logic on where to install the files that was quite spread and difficult to understand.

Fix #834 .
Related to robotology/robotology-superbuild#641 .