Vectorize slerp and remove custom code for create_group

[chrisflesher]

This makes two changes:

1. Move business logic contained in the Slerp class to _slerp function
2. Remove the business logic for create_group, let scipy handle this

This has the following benefits:

1. Code is simpler
2. Slerp can handle time input with any number of dimensions
3. Allow option to use a plain slerp function instead of a class if desired