Base SceneComponent needs some refactoring

[chrishavlin]

Various implementations have caused some drift of the baseSceneComponent class so that it now includes methods/attributes that really belong in child classes. This is most evident in the base render_gui method (link), which includes logic for colormap selection, data normalization and iso contour settings. But many components that inherit from SceneComponent should not have those settings (e.g., most of the SceneAnnotations). #176 adds a _has_colormap bandaid, but it'd be better to refactor things a bit to move child-class specific elements to their respective classes (maybe adding intermediate abstract classes).

#138 and #140 are related but focused just on the isocontours.