Tensor / TensorData API Changes

[crutcher]

Integrate plans with @laggui for Tensor/TensorData.

#156

• Tensor
  • tensor.{into,to}_data_as::<E>() -> TensorData
• TensorData
  • data.{into,to}_vec_as::<E>() -> Result<Vec<E>, DataError>
  • data.try_cast(dtype) -> TensorData
  • data.try_convert::<E>() -> TensorData

#155

• Tensor Index/IndexMut views:
  • tensor.try_index_view() -> TensorIndexView
  • tensor.try_index_mut_view() -> TensorIndexMutView