Use Specter

[mariusz-jachimowicz-83]

Use https://github.com/nathanmarz/specter for data transformations.
Why ?:

• there are many complex transfomations in LOVII schema now that could be simplified
• specter has many cool traversing/transformation features

[karlmikko]

I became aware of specter after the bulk of this development was already completed. Since then our focus has been in other areas.

Yes, I would love to move in this direction to make the code easier to read and maintain.