refactor validator

[cchwala]

Parts of validator.py still depend on deprecated Comlink objects.

Anyway, the functionality of validator classes are not used in our normal workflows. However, it could be worthwhile to have an updated validator module to have a simplified method for doing validation with gridded data or nearby gauges.

For now, I have no concrete plans for this, but it should be discussed how we can improve the validator module to do that.