Add metrics with extra non-scalar arguments

[jakirkham]

Some metrics take extra non-scalar arguments (e.g. vectors and arrays). There will need to be some adjustments both to the library and the tests to support this behavior. Have listed the metrics not yet implemented below.

• mahalanobis ( Add mahalanobis #58 )
• seuclidean ( Add seuclidean #56 )
• wminkowski ( Add wminkowski #57 )

[jakirkham]

With seuclidean and wminkowski, the functionality provided by these can be easily achieved by simply rescaling using the weights before passing it to euclidean or minkowski. So there is a fair question as to whether these are worth handling at all.

[jakirkham]

Have some WIP PRs for these three functions (mahalanobis, seuclidean, wminkowski) linked by the cross references above. These all need tests and may need some optimization.

[jakirkham]

All are now added with tests. This is complete.