binblockread breaks with USBTMCCommunicator.

[cgranade]

Using the new pure-Python implementation of USB-TMC wrapped by USBTMCCommunicator, the binblockread function now breaks for at least the Rigol DS-1000 series. This seems to be because the instrument returns the entire block at once, rather than buffering it, as is expected by the current binblockread function. My solution to this would be to move the binblockread function into the communicator class, and make the current implementation a default given by the base. I'm not sure that solves the issue, though.