Rename gain controls 'IF' and 'RF' and keep the old 'IFGR' and 'RFGR' for backward compatibility without listing them

[fventuri]

As discussed with @SDRplay and @vsonnier, we should rename the gain controls to 'IF' and 'RF', and keep the old gain reduction 'IFGR' and 'RFGR' for backward compatibility without listing them explicitly in 'listGains()'.

This change is now in the 'new-gain-controls' branch.

[dlaw]

Some references:

The original SoapySDRPlay2 issue and discussion (December 2018)
pothosware/SoapySDRPlay2#44

My previous PR to fix this in the v2 driver (November 2019)
pothosware/SoapySDRPlay2#61

A discussion about the best path forward (May 2020)
cjcliffe/CubicSDR#825

A branch of SoapySDRPlay3 which implements the changes discussed in the CubicSDR issue (May 2020)
https://github.com/pothosware/SoapySDRPlay3/tree/new-gain-controls

A port of my previous PR to the v3 driver, which I opened while unaware of the above (January 2021)
#26

New PR reconciling my proposed changes with a rebased new-gain-controls branch (January 2021)
#27

Cheers,
David