- type changed from 'const char*' to 'std::string'

[Add] 'Operator::backend()' member function and move Tensor dependence from header to source file when possible in operator

- [#include] Remove duplicated includes, add includes
- [attributes] add default values for mDataType, mImpl and mGrad
- [constructors] Order constructors, change array copy assignment to remove code duplication
- calling grad() does not instanciate the gradient anymore, a dedicated function should do it
- move getAvailableBackends() and operator=(const Tensor&) to cpp file
- Tensor::resize() now handles scalar Tensors

- [Add] ``zeros()`` member function to set implpementation elements to 0
- [unit_tests][NF] Add many more cases in Test_TensorImpl.cpp
- [include] update includes in Tensor.hpp