Absolutely/relative obstacle definitions

[mmoelle1]

• Add xmin, xmax, ... parameter to lattice structure to enable the possibility to define physical dimensions to which the grid is rescaled when exported to a paraview file. By default the grid is the unit domain.
• Add flag to the lattice structure to indicate whether object positions are absolute (i.e. in terms of grid cell positions) or relative to the physical dimension. That way it becomes easier to specify the location of obstacles for different grid resolutions.