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
Fix indexing header
Merge pull request #332 from francisco-dlp/DOC_fix_indexing_header

Fix indexing header
Merge branch '0.7.1'
BUG_FIX: Model.store_current_values was storing the values of inactive components.
Merge pull request #334 from francisco-dlp/BUG_FIX_inactive_component

BUG_FIX: Model.store_current_values stores inactive components values
FIX: missing () in visualization user guide
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
BUG_FIX: Fixes #331, FEI reader reverses: spatial dimensions
Automatic style corrections courtesy of autopep8.
Merge pull request #343 from francisco-dlp/BUG_FIX_FEI_reader

Bug fix fei reader
Merge branch 'BUG_FIX_FEI_reader' into 0.7.2
Merge branch 'master' into edslab
BUG_FIX: Fixes #331, FEI reader reverses: spatial dimensions (second attempt)
Merge pull request #339 from magnunor/ENH_plot_spectra_specify_ax

Enh plot spectra specify ax