v.1.2.0

• Fractional Difference Index (FDI) key is now both fd and nd. In order to
  avoid confusion, with Normalized Difference (NDI). They are in essence the
  same index. This is the metric by default from now on.
• We change the .png by .gif in the README.md file.
• We update the README.md benchmark section, with the new results (SUPER-IX feedback).
• We add a colab badge in the README.md file.