This PR adds 3 new functions for manipulating (custom) Quantitys:

• RunningAverage(): tracks the running average over time of a given Quantity (for each cell individually, so this function also returns a full Quantity). The averaging takes the time step into account: when advancing a time step $$dt$$, the average will be updated based on the previous average $$a(t)$$ and the value $$q(t+dt)$$ of the quantity after the step :

  $$a(t+dt) = \frac{t \cdot a(t) + dt \cdot q(t+dt)}{t+dt}$$

• Sum() and SumVector(): the total of the Quantity, summed over all cells in the simulation grid.

  • Sum() returns a scalar float64 and can be used for both scalar and vector quantities. For a vector quantity, all components will be added together.
  • SumVector() returns a data.Vector and can only be used for vector quantities.