Cleanup debian/control.in files

@adam-mercer Please, for the next release, consider cleaning up the - now outdated - dependency alternatives in */debian/control.in: libgsl0-dev is no longer present and can be fully replaced with libgsl-dev; similar for libcfitsio*-dev which is now just libcfitsio-dev. Of course, libfftw3-dev and libhdf5-dev need to be kept ;)