document and/or expose maxNyquistDealias parameter

[adokter]

vol2bird has a parameter maxNyquistDealias (currently hidden in calculate_vp/bioRad) that suppresses dealiasing whenever the nyquist velocity of all sweeps is very high. The default threshold is currently 25 m/s (90 km/h) which in most cases is below expected bird migration ground speeds (though not always). Need to document this behavior, and potentially expose it as a parameter in calculate_vp (parameter is already accessible in vol2birdR).