CMake install: reorganize for Linux

• __init__.py and other scripts go to ${destination_python}/bornagain
• libBornAgain*.py go to ${destination_python}
• _libBornAgain*.so go to ${destination_lib}

This resolves #135 (closed).

More importantly, this repairs installation to Linux, which was broken since many months. It must be left undecided when the error was introduced; bisection does not work for me because current compiler does not compile old code.

To do in new issues: We need to check whether installation works Mac and Windows.