Remove 'Tensor' from 'Operator' class. Only keep 'Data'

- Add a common parent class to each Operator using Tensors: OperatorTensor
- Gather shared operator functions in OperatorTensor
- Add generic mInputs and mOutputs attributes for OperatorTensor
- Add an enum to identify the type of Data used by each Operator
- Change Inputs, DataInputs, Outputs for Inputs, Data, Attr, Outputs for less confusion