Add "stretched" Chebyshev nodes of third and fourth kinds

[pasabanov]

Goal

Implement the following point distributions:

• "Stretched" Chebyshev nodes of the third kind: nodes are "stretched" to the right segment boundary, so that the last point lies at the right end of the segment.
• "Stretched" Chebyshev nodes of the fourth kind: nodes are "stretched" to the left segment boundary, so that the last point lies at the left end of the segment.

Note

The barycentric weights for the new distributions are the same as for the original ones, since they do not change when translating one segment to another.

Tasks

1. • Implement "Stretched" Chebyshev nodes of the third and the fourth kinds; feat(dist,bary): Added stretched versions for third and fourth kind Chebyshev nodes #57
2. • Update alfi::misc::barycentric to handle the new distributions with stable formulas; feat(dist,bary): Added stretched versions for third and fourth kind Chebyshev nodes #57
3. • Add test data; Added chebyshev(_ellipse)?_(3|4)_stretched distributions data test_data#8
4. • Update tests to use new test data; feat(dist,bary): Added stretched versions for third and fourth kind Chebyshev nodes #57
5. • Update documentation on the main website. https://github.com/ALFI-lib/alfi-lib.github.io/pull/19

Related: #47.