Postprocessing fails when there is only one datapoint.

[rajeee]

Describe the bug
Postprocessing crashes when there is only 1 datapoint.

To Reproduce
Set n_datapoints: 1

Expected behavior
It shouldn't fail.

Logs

Please copy pertinent sections of log files here (e.g. sampling.out, job.out-*, postprocessing.out, sinularity-output.log, etc.)

Traceback (most recent call last):
  File "/Users/radhikar/opt/anaconda3/envs/bsb22/bin/buildstock_docker", line 33, in <module>
    sys.exit(load_entry_point('buildstockbatch', 'console_scripts', 'buildstock_docker')())
  File "/Users/radhikar/Documents/resstock/buildstockbatch2022/buildstockbatch/buildstockbatch/utils.py", line 98, in run_with_error_capture
    return func(*args, **kwargs)
  File "/Users/radhikar/Documents/resstock/buildstockbatch2022/buildstockbatch/buildstockbatch/localdocker.py", line 299, in main
    batch.process_results()
  File "/Users/radhikar/Documents/resstock/buildstockbatch2022/buildstockbatch/buildstockbatch/base.py", line 549, in process_results
    postprocessing.combine_results(fs, self.results_dir, self.cfg, do_timeseries=do_timeseries)
  File "/Users/radhikar/Documents/resstock/buildstockbatch2022/buildstockbatch/buildstockbatch/postprocessing.py", line 280, in combine_results
    all_ts_cols.difference_update(all_ts_cols_sorted)
AttributeError: 'list' object has no attribute 'difference_update'

Platform (please complete the following information):

• Simulation platform: All
• BuildStockBatch version, branch, or sha: develop
• resstock or comstock repo version, branch, or sha: NA
• Local Desktop OS: all

Additional context
Add any other context about the problem here.