Added fast autofocus to default plugins

See merge request !6

Tidied up the fast autofocus code, including moving the bulk of the
new code from the `scan` plugin into `autofocus` and updating it to
reflect the fact that the autofocus plugin now merges both old and
new functionality (i.e. fast_autofocus is no longer a separate
plugin).  Also fixed a bug that caused it to overshoot on scans.

Since splitting the camera config into a separate YAML file, we've broken
the ability to change the camera's configuration via the API (or at any time
except when the camera is set up, I think).  This (mostly) fixes that, though
I get type errors that I've had to work around (e.g. when setting
shutter_speed), see `pi.py`

Fast autofocus in scanning

See merge request !7

Gain & exposure settings

See merge request !8

read_config was only returning keys already present in the camera's config dictionary, which meant that if the camera settings were reset/deleted, it no longer read the settings from the picamera object.  I've changed it to use CONFIG_KEYS instead, which I believe is the correct behaviour.

Temporary scan captures

See merge request !10

Fixed camera's read_config method

See merge request !11