SoapyUHDDevice: implemented DC offset and IQ Balance APIs

[ncorgan]

With how recent the IQBalanceMode changes are, I haven't included it here, but it's trivial to implement, depending on how you want to deal with compatibility.

[ncorgan]

With https://github.com/pothosware/SoapySDR/commit/349296050fc2d0d40e86b7834e9273599bfa387f commited, I'll implement those functions here tonight or tomorrow. I don't have access to the USRP I used to test previous pull requests, so I'll need you to test it.

How do you want to deal with SoapySDR compatibility? Should I surround them with the new compatibility #define? In theory, I should be able to unconditionally compile the functions, since it's an API addition and not a breakage. Older versions won't have a SoapySDR function to call into it, but since SoapyUHDDevice doesn't declare functions with override, the compiler wouldn't complain about an older version's SoapySDR device class not having the function.

[guruofquality]

The ifdefs are useful for UHDSoapyDevice.cpp when you have to call into a SoapySDR::Device inside of bound uhd functions.

You dont really need them in SoapyUHDDevice.cpp unless you have override. I say just do the simple thing and skip override and ifdef here.

[ncorgan]

As I said this morning, I don't have access to hardware to test this, so you'll have to verify.