Replace ThirdParty/Core/Faddeeva by dependence on our libcerf

this requires some ad-hoc code to work with both the C and the CPP
version of libcerf.

CMake: simplification and modernization

Extend them to CMakeLists.txt,
and bundle the checks in one CMake module.

cmake/bornagain/modules -> cmake/modules
cmake/bornagain/scripts -> cmake/configurables
cmake/generic/modules   -> cmake/extensions

CMake: remove options PYTHON2 (never again) and OPENGL (always on)

Expand include paths and guards; simplify CMake scripts

As recommended by
https://google.github.io/styleguide/cppguide.html#The__define_Guard