regroup index manipulation functions in core

As tensor values are manipulated through their indices, it is often needed to manipulate them as well.

Currently index manipulation functions are scattered across multiple files in multiple libraries.

Mostly :

• inBound
• getFlattened(/MultiDims)Index
• Retrieving corresponding coordinates of an interpolated tensor in original tensor (currently being implemented)

If you are aware of other lost index manipulation functions please signal them here.

All these functions are backend-independant and have all the common point of being all about index manipulation.

Grouping them in a dedicated file under aidge_core/include/data/Index.hpp would help ordering the code further.

I plan to do this task along  #164 when expand operator will be created.