Changed many functions in the abstract *Link classes to be const. This facilitates setting other functions as const. The metadata for specific links are reset when the links are added.

Changed NUM_OUTPUT_SIGNALS to an const unsigned short from a #define and moved its definition into OpalRT.h from OpalLink.h so that the configuration would all be in one place.

OpalLink should now be able to be disconnected.  I also added a test to see if params had already been allocated before calling new in OpalLink::connect()

Cleaned up communication structure significantly, should now be easier to add new links. Signal bytesReceived(link, QByteArray) is now only way to pass on newly received data