Imporove the performance of "sealing" the Marching Cubes when part of object lays outside the bounding-box zone

[sohale]

Also, extract the other tasks for this part:

• (including refactoring the MC code; it is too large)
• (and in general make that file more clean) #78
• (use C++20 modules (itself, after enabling C++20 modules #80 ))
• ...