Center for cell contractility calculation

[davidbhr]

Think of an option / parameter to select the mode for calculating the cell center to be used for the total contractility, either based on deformation field (probably the interpolated field) or on the force field.

Then also for time lines, if a single center from a time step (first/last) should be used for all time steps (e.g. could be used for stationary spheroids) or continuously centers (default).