Ensure output time series always have same dimensions

We currently optimize some computations by keeping constant quantities as scalars unless needed otherwise. For example, when no noises are injected into a given intereferometer the corresponding signal will just be a scalar 0, not a numpy array filled with zeros. This propagates all the way to the output file.

This is probably not optimal, as the output file format should not depend on the instrument configuration, since that potentially breaks (or at least complicates) interfaces to other tools.

Proposal would be to always broadcast the final output time series to the expected shape, even if they are constant.