Fortran `*.mod` files need to be installed in the `include/` directory

[sloede]

Right now we put them in the lib/ dir, because I guess this is how we conceptually think about them. However, they really belong into the include directory, since that's where gfortran/ifort will look for them.

[benegee]

I just did not know. And as they are in a binary format I opted for lib. 😄