First pass at supporting multiple views.

We still need to handle deletion of views. Also, we can organize the
modules a little better to make it easier to work with mutliple views.
Cache the histogram values as calculated

This makes it much easier to switch between several data sources,
after the initial binning everything is fast.
Added threshold module -- although we shuoldn't really use it.

It's very slow!!
Added axes/camera toolbars.
Set a marker, make the bars blue with black outline

Make it obvious where the current contour value is, improve the look
of the contours a little.
Merge branch 'histogram'