Put mor emphasis on the fluid grid and snapping of VENT/OBST

[TristanHehnen]

We should be more careful in explaining how the mesh influences the size and location of objects in the simulation. Like how things VENT or OBST snap to the mesh and that this may lead to different actual sizes compared to what was written in the input file.

Also wall thincknesses in terms of SURF and OBST, and what it means when multiple OBST touch each other. Like when someone would try to replicate some sort of sandwich material or wall lining.