Make it possible to specify multiple OBSERVATIONS keys in config

From Sondre
_The input from the discussion was that it should be possible to split your observation definitions in multiple files (similar to what you can do with parameters). So technically we should allow using the OBSERVBATIONS keyword multiple times, and concatenate the lists of observations together_

Locate the configuration parser for the OBSERVATIONS keyword and compare it with the existing handling of repeated parameters. Trace how observation definitions are collected, then inspect nearby tests or add coverage for multiple OBSERVATIONS entries whose lists are concatenated. Done means separate configuration files can define observations through repeated OBSERVATIONS keywords without losing entries.

Written by the indexing model from the issue text.