BUG: plot_spectra() does not correctly plot a decreasing axis

[jat255]

If the scale of the x-axis is negative (common in IR spectroscopy), using the plot_spectra function will plot the data in increasing order:

Example plotting using s.plot():

Example using plot_spectra():

This is because matplotlib defaults to an increasing axis, and there is no explicit setting of axes limits like there is in signal.plot() (see https://github.com/hyperspy/hyperspy/blob/RELEASE_next_minor/hyperspy/drawing/signal1d.py#L113-L116).

This shouldn't be too hard to fix by copying those lines or similar into plot_spectra

[ericpre]

Fixed in #1973.