Tune_request feature for UHD's USRP blocks in GRC

A few people have mentioned wanting to tune the UHD’s USRP source/sink
block without any tune error correction in the FPGA DSP. Described here:

So you can do this in python with tune_request as follows:
tr = uhd.tune_request(target_freq)
tr.dsp_freq = 0
tr.dsp_freq_policy = uhd.tune_request.POLICY_MANUAL

However, this is cumbersome to setup the struct inside GRC. So, I added
a convenience constructor that initializes the struct using **kwargs:
Inside the tune frequency parameter, use this one-liner instead:
uhd.tune_request(target_freq, dsp_freq=0,

If you want to use this feature, go ahead and cherry-pick this change. I
tested out that the constructor works but have not yet given it a live