uhd: added a usrp PC sync option
uhd: added getters for bandwidth setting and range
uhd: Use the stream arg setting to correctly map the channels

Currently we have been assuming channels are linearly mapped 0, 1, 2...
reguardless of how the user has set the stream arg channels.

This change makes channel n on the gr-uhd wrapper
use the nth channel as set by the streamer channels.

Using this, a user can properly use multiple usrp source or sink blocks
for a single device and have each block correspond to a particular
set of channels as defined by the streamer channels parameter.
uhd: added uhd blocks param for streamer channels
uhd: fixed set_center_freq channel
uhd: default num channels when empty
uhd: only issue stream cmd on selected channels
uhd: and also actually use said stream command
gras: fix from last nights swig fun