Update operators implementation

See merge request !22

- 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'

Add Scaling operator

See merge request !21

add sqrt operator

See merge request !18

# Conflicts:
#   include/aidge/backend/cpu.hpp

Binary operators

See merge request !19

Update with default operator impl

See merge request !17

Added requirements.txt

See merge request !15