Changed three_sample_window to operate on unmasked slices.

Will avoid last_value being carried on between masked sections of data.
Merge branch 'master' of https://github.com/FlightDataServices/FlightDataAnalyzer
Fix for gross weight name errors.
Switch AirspeedRelativeFor3Sec to use new three_sample_window.
Fixed second_window and switched to using it in params.
Merge branch 'master' of https://github.com/FlightDataServices/FlightDataAnalyzer