meta issue of sprint goals

• graphic features docs #197
• Histogram vmin vmax widget  #166
• auto-generate mixin class for Subplot to hold all the add_<graphic> methods #157
• refactor ImageCmapFeature #132
• refactor and write tests for more cmap handling, add cmap_values #241, the cmap_values parsing code is messy right now
• fix broadcasting errors when changing data in ImageWidget to different shape
• more camera and controller options, pygfx has many more options now, like FlyController
• Nicer readme with more epic visualizations, ideas from demo ideas #9