get_significant_duration improvement

[jorge243]

this should be faster and i think improves readability:

def get_significant_duration(acceleration, time_step, start_level=0., end_level=1.0): """ Returns the significant duration of the record """ assert end_level >= start_level husid, time_vector = get_husid(acceleration, time_step) idx = np.searchsorted(husid, [start_level * husid[-1], end_level * husid[-1]]) return np.diff(time_vector[idx])[0]