Textgeom relative paths

[werthm]

These changes allow to specify geometry files via the #include directive by using relative paths. Example:

geometry/scenario_1.dat
geometry/structure_1.dat
geometry_structure_2.dat
simulation/geometry_1.dat

Content of geometry/scenario_1.dat

#include structure_1.dat
#include structure_2.dat

Content of simulation/geometry_1.dat (Working directory)

#include ../geometry/scenario_1.dat