Variant Record decoding for RPR communications transport does not set values properly

[timpokorny]

Summary
I noticed when debugging an issue with the conversion of Transmitter that the RPR framework is not managing variants properly. When we receive a PDU and go to turn it into the RPR version, we are not always setting the discriminant. We often make the call to setVariant(), but this only sets the value of the variant - it doesn't "activate" it by changing the current discriminant as well.

Check what we are doing in RadioTransmitter.fromPdu() now around the Major RF Modulation Type. This one is now calling setVariant and setDiscriminant.

https://github.com/openlvc/disco/blob/master/codebase/src/java/disco/org/openlvc/disco/connection/rpr/objects/RadioTransmitter.java