Custom Networks + Rarity Algorithms + Elrond Network config

[johnykes]

This PR comes with the following features:

• support custom network/marketplace customizations (file paths/names, metadata format/type);
• support rarity computation with the most used rarity algorithms (Jaccard Distances, trait rarity & statistical rarity);
• add elrond network preferred metadata format;
• code improvements.

PS: all the utils/scripts were updated in order to support these changes.