- 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

Changed outputs() and getNbDataInputs() in GraphView to achieve consistent behavior (clarified in doctring)

- split Input into Data and Attr
- change inputData for Data
- remove getInput/getOutput and input/output member functions
- remove member functions refering to Tensor dimensions