Double suffixes when saving plots as png, eps, etc

[jameswilburlewis]

In tplot we have:

    if save_png is not None and save_png != '':
        plt.savefig(save_png + '.png', dpi=dpi)

and similar for other output formats. We should check whether there's already a suffix before adding '.png' or whatever, to avoid filenames like myplot.png.png.

[nickssl]

The same problem appears in pyspedas:

mms_orbit_plot.py mms_fpi_and_ang.py (this is somewhat different) slice2d_plot.py

[nickssl]

Fixed.