For now only strings are accepted as the measures parameter in GridSearchCV, RandomizedSearchCV, and cross_validate. It's thus impossible to use those with measures that take specific parameters as input (e.g. #156 ), or to use custom measures.

We should then accept callables in addition to strings.

Each callable should only take the predictions parameter. In order to handle measur

See:
gonum/gonum#845 (comment)

In ncollide we have an IsometryOps trait that defines the lerp_slep method for isometry interpolation. This interpolation uses linear interpolation for the translation part of the isometry, and spherical interpolation for its angular part.

We should add this lerp_slerp method directly to nalgebra's `Is

Sytests:

    × Setting state twice is idempotent