code: def test_std(self):
      np.testing.assert_almost_equal(calc_std(const0), 0.0)
      np.testing.assert_almost_equal(calc_std(const1), 0.0)
      np.testing.assert_almost_equal(calc_std(constNeg), 0.0)
      np.testing.assert_almost_equal(calc_std(constF), 0.0)
      np.testing.assert_almost_equal(calc_std(lin), 5.766281297335398)
      np.testing.assert_almost_equal(
          calc_std(lin0), 6.069769786668839, decimal=5
      )
      np.testing.assert_almost_equal(
          calc_std(wave), 0.7071067811865476, decimal=5
      )
      np.testing.assert_almost_equal(
          calc_std(offsetWave), 0.7071067811865476, decimal=5
      )
      np.testing.assert_almost_equal(
          calc_std(noiseWave), 0.7128230620125536, decimal=5
      )
  
imports: import unittest
  import numpy
  from tsfel.feature_extraction.features import *
label_desc: Use image node_id 9 for calling the TestFeatures obj's underlying member method code with example usage: obj.test_std() without return types