[FEA] Improve the iface struct and its associated free functions

[viclafargue]

Is your feature request related to a problem? Please describe.
The SNMG ANN PR introduced in addition to the MG features a standardized API to facilitate the use of ANNs. It exposes an iface struct storing the index and some free functions to build, extend, search and serialize the index. This standardized API is in its initial state, but should be refined over the next releases.

Describe the solution you'd like
The details would have to be discussed during the next development process.