More improvements

1. Previously if you tried to call an ECR::set_tx...() function while a frame was being transmitted the function call would hang until the transmission was ended. This was done for thread safety but it caused hang ups. Now these functions modify an intermediate struct and the transmitter is updated as necessary between transmissions.
2. Depending on your configuration and sampling rates you can run the risk of saturating the processor. This results in overflows on the USRP receiver side and/or underruns at the transmit side. I've directed the relevant UHD messages so that they will trigger a CE event and can be dealt with accordingly.