Issue with Vector6d declaration

Job #55085 failed for gsaurel/curves@a1ed4698c4a6377a1b2910ce7c4ace66a10cb2e7:

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ERROR: test_piecewise_se3_curve (__main__.TestCurves)
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Traceback (most recent call last):
  File "…/curves-0.2.0/python/test/test.py", line 621, in test_piecewise_se3_curve
    self.compareCurves(pc,pc_txt)
  File "…/curves-0.2.0/python/test/test.py", line 33, in compareCurves
    self.assertTrue(norm(c1.derivate(t_min, 1) - c2.derivate(t_min, 1)) < 1e-10)
TypeError: No to_python (by-value) converter found for C++ type: Eigen::Matrix<double, 6, 1, 0, 6, 1>