Dummy device: model single device operation

Add a configuration option 'single' to the template for the dummy device.
When this is set to 'yes', yield a single device rather than an odd/even
pair.

This is the beginnings of pulling through single-camera support in spreads - running with this configuration in place gives the error

    There is a problem with your device configuration:
    Please connect and turn on two pre-configured devices! (1 were found)

but it means I can work on sorting out the issues in the core code without having to have a camera connected.