Read a marker file for advection

In order to probe the gas-flow in the computational domain a density marker field can be advected with the hydrodynamic velocities. First the advection parameter in the parameterfile has to be set to 1 and an initial marker field has to be provided. The file provides simply an integer with the value 1 at the location of the marker-field. In the simulation a marker density will be assigned to the marker-field regions, which will than be advected.