change numpy matrix usage in wx2d (radialvel)

[SaOgaz]

This should eliminate the following pytest deprecation warnings

tests/test_wx2d.py::TestWx2d::test_wx2d_t1
/Users/ogaz/stistools/stistools/radialvel.py:226: PendingDeprecationWarning: the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.
m_a = N.matrix(a)

/Users/ogaz/stistools/stistools/radialvel.py:227: PendingDeprecationWarning: the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.
m_target_j2000 = N.matrix(target_j2000)

/Users/ogaz/miniconda3/envs/stistoolsdev/lib/python3.6/site-packages/numpy/matrixlib/defmatrix.py:68: PendingDeprecationWarning: the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.
return matrix(data, dtype=dtype, copy=False)