Moved symbols into main lib

This fix should address #6 (closed) but since @jvinyals is not here to test it, I still would like to merge it soon.

How it fixes the #6 (closed) issue: The static library (sit_f) that made problems is no longer there by default. Just linking with libsit.so will be enough, as I include the Fortran symbols now in that lib. I adopted the tests and they still run.

I also added the option BUILD_FORTRAN_LIB to enable the building of the library, as some HPC machine have problems with mixed language libraries.