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

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.

Locate the lecture or exercise notes covering the mesh and the VENT, OBST, and SURF keywords. Read how object dimensions and positions are currently explained, then document snapping, effective sizes, wall thickness, and touching OBST objects; done means the material clearly addresses these cases for someone writing simulation input.

索引モデルが issue の本文から書いたものです。