[Discuss-gnuradio] SDRplay rewrite w/RSP2

[Jeff Long]

I picked up a SDRplay RSP2 recently, and the gr-osmosdr support needed 
some work. Here's a rewrite that works with RSP2 and has other 
improvements. When the RSP1A comes in, I'll finish support for that. No 
plans to pick up a RSP1, but someone can see if this works if 
interested. This was tested against the Nov 2017 binary blob.

https://github.com/willcode/gr-osmosdr/tree/sdrplay2

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Complete rewrite of sdrplay support

- Supports RSP2
- Will support RSP1A when hardware arrives
- RSP1 is untested
- Uses streaming callbacks instead of polling
- Tuning, gain control imporoved, AGC supported
- Better performance (one less layer of buffers)