Method to update simulation metrics

[gmaze]

Let's consider the following scenario:

• On one day, we run a simulation for the next unknown cycle number CYC. The json output is generated without the observed cycle as reference, hence without prediction error metrics.
• On some later day, we want to re-examine simulation of cycle CYC. At this later day, the real float has completed cycle CYC.

Since we now have the observed reference cycle, we should have an 'updating' method in order to modify the json output file with the prediction error metrics.