fix matplotlib `Figure already exist` error while running view in parallel via smallbaselineApp (#1503)

+ view.py:
   - set matplotlib.use('Agg') BEFORE importing pyplot, and switch backend in `viewer.configure()`.
   - properly close fig object
   - do NOT specify the `num` while calling subplots(), to avoid potential conflicts between parallel processes of view.py calls

+ smallbaselineApp.plot_results(): use `multiprocessing` backend while calling joblib.Parallel()