USB communication changes

[Jef Driesen]

Hi,

Native USB communication (used only by the Atomic Aquatics Cobalt backend) was 
not yet ported to the I/O stream interface. This has now changed because I just 
pushed a new USB based I/O transport implementation the libdivecomputer master 
branch. This means the application is now responsible for setting up the USB I/O 
stream, just like all others transports. The Atomic Aquatics Cobalt backend does 
no longer open the USB device internally. Passing a NULL pointer for the 
iostream will no longer work!

The advantage is that an application can now also use a custom implementation on 
platforms where libusb isn't available (iOS) or doesn't work very well due to 
permission problems (Android).

Jef