Set start_time when using a frequency_array

When using the

interferometer.set_strain_data_from_frequency_domain_strain(strain, frequency_array = freqs, start_time = gps_start_time)

function to set the data in an IFO, ie setting the frequency_array directly from an array, and not specified through sampling_frequency and duration, the start_time parameter isn't initialised properly. Instead it's set as (the default of) start_time=0.

This MR provides a fix which solves this specific problem for the run I set up, but this could potentially have some deeper cause that I've missed... (note that I've only tested this for FD strain data, and not yet for TD strain data input)