Support for any backend storage

[MAJ][IMPR] making storage references a future_std::any in order to support any backend. [TODO]: some implementations supports only CPU. Must be fixed or protected against access to other backend. [FIX][UT] Also fix some warnings in a unit test closes #21 (closed) closes #35 (closed) related #42 (closed) related #45 (closed) related #46 (closed)

NB !13 (closed) should be merged first