Install SWIG interfaces to CMAKE_INSTALL_PREFIX/language by default

[jaeandersson]

The default installation directory for e.g. the Python module should be CMAKE_INSTALL_PREFIX/python, for Octave CMAKE_INSTALL_PREFIX/octave etc. This avoids accidental installation into system paths and makes it easier to switch between multiple versions of CasADi.
Cf. #1982

[jaeandersson]

Both MATLAB and Octave will be installed into CMAKE_INSTALL_PREFIX/matlab, as they should not conflict and have a lot of common code.