utils.py: add function plot_same_navigation

Can plot several spectra which all share the same axes_manager.
Making it possible to change the navigational position of all the plots
at the same time.
BUG-FIX: utils.plot_same_navigation had wrong variable name
utils.plot_same_navigation: improve docstring
Modify plot_same_navigation: add navigation parameter, rename function

Add navigation parameter which is passed to the individual plot
functions.
Renamed function to plot_sync_navigation
plot_sync_navigation moved from utils.py to drawing/utils.py
Merge branch 'master' into plot_same_navigation

Conflicts:
	hyperspy/drawing/utils.py
	hyperspy/utils.py
utils/plot.py: add drawing/utils.plot_sync_navigation to import
utils.plot.plot_sync_navigation: add preliminary docs in user guide
Change plot_sync_navigation to plot_signals

Add argument sync, which if set True will let all the signals share
the same navigation positions. If set False all the signals will
have seperate navigation positions.
Change documentation to plot_signals (instead of plot_sync_navigation)
Fix typo in plot_signals documentation
Merge branch 'master' into plot_signals

Conflicts:
	doc/user_guide/visualisation.rst
Modify drawing.utils.plot_signals: add slider argument, navigation_list

navigation_list argument makes it possible to specify different
navigators for the different signals.
user_guide.visualization: Remove residual after using mergetool
Make plot_signals work when the signals have different signal shapes
BUG-FIX: the image explorer was not updating the image when plotting using an external axes_manager.
Merge pull request #1 from francisco-dlp/ENH_plot_signals_different_signal_shape

Make plot_signals work when the signals have different signal shapes
Modifications to drawing.utils.plot_signals

Will now raise an ValueError if navigator_list and signal_list is
not same size.

navigator_list will be now be used if sync=False.
utils.plot.plot_signals was not working when navigation_list was None
Improve user guide documentation for plot_signals