EHN: background for stack of image.

Test in plot works but not plotting
EHN: Docstring for top_hat, link_standard, k-ratio
EHN: add option in plot()

navigator False, spectrum/image. Work fine in 2D. not in 3D. Prob of pointer
Merge branch 'ENH_3D_image_plot' into ENH_background_for_plot

Conflicts:
	hyperspy/signal.py
Merge branch 'ENH_background_for_plot' into NEW_edsSpectrum
Merge branch 'NEW_edsSpectrum' of https://github.com/hyperspy/hyperspy into NEW_edsSpectrum

Conflicts:
	hyperspy/signals/eds_sem.py