Mark snglcoinc_verify.py as XFAIL when required

Description

This MR updates the LALBurst snglcoinc_verify.py test as follows:

• exit with code 77 (SKIP) if lalburst.snglcoinc can't be imported (when python-ligo-lw isn't present)
• mark as XFAIL when python-ligo-lw is present alongside scipy < 0.19.0

This will allow the failing test to be reported, but not in a fatal way, until #208 (closed) can be addressed.

When #208 (closed) is addressed properly, c296c43b (or it's equivalent on lscsoft/lalsuite@master) should be reverted.

/cc @kipp.cannon, @adam-mercer

API Changes and Justification

Backwards Compatible Changes

• This change introduces no API changes
• This change adds new API calls

Backwards Incompatible Changes

• This change modifies an existing API
• This change removes an existing API

Review Status