Refactor timemap

This series refactors the TimeMap class with the intention to use it as a replacement of opm-core's SimulationTimer. Altogether this PR is rather large, but the individual patches are rather small and the PR is complete AFAICS. If you would rather have bite-sized but not fully complete PRs: No problem, I'll just open PRs for the individual patches (they all have been tested insofar that it compiles and after each patch and the tests still pass). If you require me to do bite-sized and complete PRs, I'll be off for a while to research the reality distortion field a bit ;)

note that this PR also contains #127