refactor spectrum assignment
refactor spectrum assignment

use the new function compute_spectrum_slot_vs_bandwidth

Signed-off-by: EstherLerouzic <9136e9bbec0a09a1685b8ebbe677b6c9b26e8f18@orange.com>
Change-Id: If045fe53550dbde57c56fd4e8b99275c0757ea2b

Telecominfraproject/gnpy