Fix generation of simulated data when using the PE pipeline
As reported by @zachary.metzler, when using the cwinpy_pe_pipeline
with a configuration file that asks for the fake data to be generated from the design PSD of a single detector, it fails with an error ending with:
File "/cvmfs/software.igwn.org/conda/envs/igwn-py310-20230711/lib/python3.10/site-packages/cwinpy/pe/pe.py", line 2011, in <setcomp>
"{}:{}".format(det, simdata[freqfactor][det])
TypeError: string indices must be integers
This MR fixes this issue, which occurs due to the configuration parser not correctly dealing with cases when a single detector string is given for the fake-data
arguments.