Resolve "Operator Attributes consistency"

Context

Closes #237 (closed)

Use X-macros to keep Operator attributes enum, names and type in synch.

Main modifications

• add: list unrolling macros to Operator.hpp
• add: X-macro for attributes management of every Operator with static attributes.
• add: [experimental] some attributes enum were coupled with the related class. A prototype is also available for templated classes (Conv, Gather, Heaviside, LRN, LeakyReLU, Memorize, ReduceMean, ReduceSum, Transpose)
• upd: make attributesName() static member function to allow its definition in header outside of class declaration while avoiding 'multiple declaration', error