add feature weights (w-k prototypes)

[joaquin-tempelsman]

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Based on proposed approach by Çörekci, Osman, and Ayla Saylı. "A New Clustering Algorithm of Hybrid Data According to Weights of Attributes." Avrupa Bilim ve Teknoloji Dergisi 5.9 (2016): 28-37.

Implemented feature adds feature weights by passing a weight argument per categorical and numerical feature. This weight is used by both default similarity functions (euclidian for numerical and matching for categorical). Assertions checking for correctly passed weights are added under init and fit methods.

Pending: include integration tests