add offset to parameter queries at intersections

This closes #20 (closed) and #22 (closed).

Added an offset to the parameter queries via global coordinates at grid intersections. This avoids querying the one and same value of the underlying parameter field if the grid intersections coincide. Thereby, the parameter query honors the weighting of the conductivities at both sides of the intersection.

Cleanup: Removed some repetitive calls to geometries and iterator positions.

Edit To-Do:

• Create new references for testing