Make ConceptTimeValuesTable stateless

ConceptTimeValuesTable really only has one task: to compute the table from a list of concept keys. This commit changes the implementation to only use a single method and not use any additional state in the object. Less moving parts = less bugs.

I noticed this when reviewing carlos' pr for the tests for ConceptTimeValuesTable. This pr is intended as much as a discussion piece as as a code change, so do you think such a less statefull design is better or not?