Brainrender

Major changes, check the docs for details: https://docs.brainrender.info/usage/overview/v1-greater-than-v2

**New**:
- added a new `Actor` class which makes it easier to manage how `Render` transforms actors at render time. `Actor` also create's labels and can be printed to get more details about a specific mehs
- exposed `scale` for screenshots (thanks vigji )

**Bug fixes**:
- multiple calls to `render` now transform neurons correctly
- `scene.add_brain_regions` now returns colored brain regions when only an hemisphere's mesh is requested

**change log**
- Fish neurons API
- compatible with python 3.7
- allensdk optional
- new axes look
- fixed axes origin

*small stuff*
- improved `camera` stuff
- new scene special attributes
- scene.list_actors