Don't import lal at module level in igwn_segments.utils

Currently the igwn_segments.utils module cannot be used on systems that don't have LAL (e.g. Windows) because there is a module-level import from lal.

I would like to see the module level import refactored by the appropriate function-level imports where necessary, so that the other functions in igwn_segments.utils can be used on LAL-free systems.