Bug when writing a measurement file with no GW file input

Playing around with LISAInstrument, I am thrown an error when running a simulation without GW inputs.

AttributeError: 'Instrument' object has no attribute 'gw_group'

changed milestone to %v1.2

added bug label

created merge request !128 (merged) to address this issue

mentioned in merge request !128 (merged)

mentioned in commit 1f895a5b

closed with merge request !128 (merged)