[fix/rawPtr][MIN][DSGN] encapsulating raw pointer access into an helper function in order to make it easier to change implementation related to aidge_core#21 and related to aidge_core!41 (which will get easier to merge threafter)

- adapt to core changes with Operator not refering to Tensors anymore
- remove assertions already performed in abstract operators constructions
- fix missing 'template' keyword prior to dependent template name 'dims'