[ENH] add and refine Transform+Riemannian distance+weights+knn

[futuer-szd]

1. add Burg-type Nuttall Strand algorithm in _nuttall_strand.py, this is a power spectral density matrices transform
2. add a simple test for _nuttall_strand.py above
3. add function to calculate optimal weights for riemannian density matrices in _get_optimal_weights.py
4. add normal pairewise riemannian distance function in _riemannian_matrix.py
5. get _check_inputs and _is_hpd function back to test_riemannian_matrix.py
6. change a link import for _is_hpd function in test_power_spectrum.py
7. add a special knn in _nuttall_knn.py for the 3D input which get from NuttallStrand transform

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