USB ports always probed when 'finesse.hardware.serial_device' module loaded

[alexdewar]

The reason for this is that the list of USB serial ports is passed as a class parameter for SerialDevice. While this is fairly harmless, it is a bit of a code smell. It's daft that we probe USB ports even when just generating documentation, for example.

[alexdewar]

One possible solution for this it to allow device parameters to be specified as Callables, to be lazily evaluated later. (The downside of this approach is that I think if we do this, we'll have to wait until an instance of the device class is created before evaluating the function, rather than just doing it as the program loads.)

It makes sense to wait until we've done #640 before tackling this, as that will also likely involve modifying the way we deal with serial devices.