Fixed DraggablePatch picking

Previously, there was no mechanism for setting a DraggablePatch.picked
to False when it was no longer picked.
DataAxis bugfix + value2index takes ndarray

Fixed a bug in DataAxis._get_name() when the axes_manager was not set.
Also, DataAxis.value2index() now accepts ndarrays as input.
Gaussian peak value helpers

Adde helper property to Gaussian which sets and gets the function value
at the peak. It is simply parameter 'A' scaled by sigma*sqrt(2*Pi).
Bugfixes to selective component update_plot()

Fixed an oversight in parameter connection/disconnection, and fixed an
error when trying to update component plots when they are not enabled.
More resilient CmpPartial

No longer throws an exception when comparing to objects without the
correct attributes.
Made addition/removal of Model components dynamic

Made Model.append() and Model.remove() play with
Model.enable_plot_components() and Model.enable_adjust_position(), so
that the plot is automatically updated if adding or removing.

Conflicts:
	hyperspy/model.py