Improve camera logic

[sibocw]

See #185, #187.

Specific issues:

• Unclear what is_visualization_camera indicates
• Unclear why neck_zoomin camera is treated differently from, eg. head or tarsus cameras
• Checking if the camera is a camera defined in the XML (I think this is what it does?) by checking if the name contains "Animat" is a bad idea — in multi-fly simulations, cameras attached to different flies might have names that start with different strings because the flies might have different names.
• Why does the camera have to be attached to a fly? Shouldn't this be optional in case of static cameras attached to the arena?

This is not urgent.