some ideas from #85 and more

Added:

• added CmapFeature which basically wraps ColorFeature and just provides a __setitem__ that parses a user provided str cmap to a [n_points, rgba] array using n_colors based on the slice
• added ImageCmapFeature
• ImageDataFeature which handles ImageGraphic data as as pygfx.Texture
• PointsDataFeature which handles data for LineGraphic and ScatterGraphic
• BaseGraphic which just parses the class name in __init_subclass__ and adds a string type class attribute which is used by Subplot to add the various add_<graphic_name> methods.
• Graphic has a position property which is basically just world_object.position

Modified:

• Graphic doesn't instantiate any of the graphic features now, it just handles name and a few other thing and some methods. The individual graphics now handle their features like data, colors, cmap, etc.
• ImageGraphic uses vmin, vmax now instead of clim, update ImageWidget accordingly.
• ImageGraphic uses TextureView to set geometry.grid instead of Texture, this allows setting the filter argument to Texture.get_view(filter=...), default is "nearest"
• ImageGraphic cmap setting is now functional, it uses a ImageCmapFeature instance to manage it.
• Subplot.add_graphic sets the z_position based on the number of Graphics in the subplot
• removed utils.fix_data and utils.to_float32, these are now in the data features classes or module