Low bit support

This MR add new implementations and features to support low bit export on devices based on Arm cortexM processor.

• Add configuration files for STM32F7 in the available boards

• Add implementations and custom datatypes to support low bit inference Custom datatypes :

• data<7> (int7) & udata<7> (uint7)

• data<6> (int6) & udata<6> (uint6)

• data<5> (int5) & udata<5> (uint5)

• data<4> (int4) & udata<4> (uint4)

• data<3> (int3) & udata<3> (uint3)

• data<2> (int2) & udata<2> (uint2)

• data<1> (binary)

• Add registry for low bit implementations in operator.py

• Add jinja templates for lowbit implementations

• Add data_conversion map for ARM cortexM lowbit export in data_conversion.py. This map is passed as argument in the instanciation of NodeExport in the registry for low bit implementations in operator.py.

Closes : #22