def fai(domain, b, threshold=None):
    if threshold == None:
        threshold = float(domain.algorithm_params['fai_threshold'])
    return get_fai(b).lte(threshold)
Floating Algae Index. Method from paper: Feng, Hu, Chen, Cai, Tian, Gan,
dataset/ETHPy150Open nasa/CrisisMappingToolkit/cmt/modis/simple_modis_algorithms.py/fai