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]