enhancement: add DVB RF measurement

[cyber5tar86]

I saw that the VBER has been implemented; I thought it would be useful for DVB to add other RF measures such as:

• Signal Strength
• C / N
• PreBER (BER before Viterbi)
• MER
• PER
• FER
• Signal Quality
• Constellation Diagram

inspired by this product http://www.decontis.com/components/sambuddy-rf/

I would add sambuddy-rf features as "desiderata" or "todo" in the roadmap of leansdr

[pabr]

Signal strength, MER and C/N are displayed with --gui and exported to third-party user interfaces through --fd-info. C/N computation must be enabled with --cnr. The constellation is displayed with --gui and exported through --fd-const.

I am not familiar with the other metrics.

The purpose of leandvb is to demodulate efficiently on low-cost hardware for amateur radio applications. This requires many trade-offs: metrics are computed on a subsampling of the input signal, MER is not reliable at low SNR, values are occasionally converted to fixed-point, etc. The measurements are provided for troubleshooting only.