Refactoring Tensor

Adding context to Tensor
Adding "active area" to Tensor
Forward declaring TensorImpl in Tensor instead of the another way around
CPU storage is unsigned char instead of void *
todo: memory location for device different than CPU
todo: a Tensor can share a TensorImpl with another Tensor
TensorImpl = storage
Tensor = view on storage

Notes:

• Tensor::getImpl() returns a reference not a pointer, thus client code must replace getImpl()-> by getImpl().
• Tensor.hpp does not includes backend\TensorImpl.hpp but backend\TensorImpl.hpp includes Tensor.hpp. Thus client code needs to include backend\TensorImpl.hpp (with or without Tensor.hpp)