Enhance typing of 'Model' and 'Vector' using 'typing.Generic'.

- Make `Vector` a `Generic[T]` to indicate that stored data arrays
  should have coherent types.
- Make `Model` a `Generic[VectorT]` to indicate that a given model
  class is associated with a unique vector class.
- In `Optimizer` and plug-ins ABCs, specify that inputs and outputs
  should be coherent using `Vector[T]` and `Model[Vector[T]]` type-
  hints.
- In concrete plug-in classes, leave the existing code as-is so as
  not to make it harder to read.