Modify drawing.utils.plot_spectra: support for specifiying a subplot

Currently only works for cascade, but should be easy to make for some
of the other styles.
Modify drawing.utils.plot_spectra: add ax-parameter for 'overlap'

Also add a sentence in ax-parameter docstring
Merge branch 'master' into ENH_plot_spectra_specify_ax
Merge branch 'master' into ENH_plot_spectra_specify_ax
ENH: add plot spectra ax argument description to user guide
ENH: fix wrong image in plot_spectra user guide
Removed typo fix code commited elsewhere
Merge pull request #339 from magnunor/ENH_plot_spectra_specify_ax

Enh plot spectra specify ax