gphoto2 compatibility

[hcwinsemius]

Enable operation of the rig with gphoto2. The assumption is the following:

• We still have one child pi per device. Connect one camera to each child.
• We don't need additional SD Cards in the gphoto2 compatible devices.

This will require some work and testing on the following.

• refurbishing the class (see camera360gphoto.py) that operates a gphoto2 device over USB (instead of a raspi camera over raspi cam connection)
• testing how photos can be stored in a buffer instead of on another sd card inside the camera
• Connecting the gphoto2 class to the tasking system in __main__.py
• Make a configuration option in the parent that selects the camera type you are using, and make the child responding to that by selecting the right class. This can be done in the first task sent by parent to child (see camera360rig.py, which includes all tasks), that brings the child state from idle to ready called in the function task_idle_to_ready.