Make it possible to add annotations to plots

[rubenthoms]

Make it possible to add annotations as e.g. horizontal/vertical lines or points to plots.
Good starting point: SimulationTimeSeries module

Example use case:
Production rates plotted together with capacities, f.ex oil, gas, liquid, water and water injection in the same window. A simple solution could be the option to add constant lines, or at least be able to make constants in the vector calculator)