Add option to split free gas phase into "moving" vs "stationary"

This is an attempt to quantify how much of the CO2 that is moving vs stationary. In can be useful when comparing across many realizations. It is related to structural trapping of CO2 (but it is not the same).

We assume that we are most interested in splitting the free gas CO2, since dissolved and residually trapped CO2 is already trapped (although dissolved co2 can still move).

Try this implementation first: For each time step, calculate how much of the free gas co2 that has moved (changed grid cell) in the last 25 years (or some other number of years). To avoid time step dependecies, we interpolate the state at 25 years earlier when this is not an existing time step (so with time step 2100-2120-2140-2160, calculating this for year 2160 would mean interpolating years 2120 and 2140 to get the state at year 2035).

We already have some test code in the branch CCS-447 in ccs-scripts.

We need to do calculations in ccs-scripts, resulting in changes to CSV-files for CO2 mass. So we also need to update the plotting code in CO2Migration to handle this. Maybe allow the user to choose in CO2Migration is co2 free gas should be split or not (given that we have calculated the split).