F7 Frequency/Duty Cycle not right

[ctacke]

Using the new 2.x frequency and duty cycle measurement features on the F7.

Instantaneous frequency measurements seem fine (tested 1Hz to 100kHz).

However, if I set input duty cycle (DC) to 50%, the API is giving me back 0.0 until about A 6Khz, at which point it shifts to giving sometimes ~0.53 and sometimes still 0.0.

The average frequencies are also incorrect. I get 0.0 until the input gets above about 2kHz. Even at that point, the reported average is ~7kHz.

There is a sample app in PR state right now:
https://github.com/WildernessLabs/Meadow.Samples/pull/57/files

And the Core bits that make the call are also currently in a PR:
https://github.com/WildernessLabs/Meadow.Core/blob/373c047469f459766c5ece003433a3e03651848d/Source/implementations/f7/Meadow.F7/Devices/F7DigitalSignalAnalyzer.cs#L68