modified scripts/plot_proftrace.py to close figures after saving to file

[BrettHoover-NOAA]

Minor changes to scripts/plot_proftrace.py:

1. Close each figure after it has been created and saved to file. In the absence of these plt.close() commands matplotlib.pyplot will raise a warning if more than 20 figures are opened without being closed. We don't need to keep these open after they have been written to file, so it's fine to include these close commands.
2. Replace checks on input types from if type(x) != list to the now standardized if not isinstance(x, list) format, for pycodestyle

[kevindougherty-noaa]

Looks good to me.