update docs to better describe level-set special cases

   EB2::CylinderIF my_cyl(radius, height, direction, center, inside);

   auto gshop_cyl = EB2::makeShop(my_cyl);

2. Construct the implicit function representing the EB seen by the particles.

   This only deviates from the "standard" EB by adding additional walls to

   Mass-Inflow boundary conditions. This is necessary in order to have the

   "correct" volume fraction used by the

   :cpp:`MFIXParticleContainer::PICDeposition` function. I.e. this function

   needs to see the mass-inflow as a volume fraction of 0.

2. (Optional) Construct the implicit function representing the EB seen by the

   particles. This might deviate from the "standard" EB depending on the

   specific application. In all standard cases used by mfix, this step is

   omitted.

3. Call :cpp:`mfix::build_eb_levels(gshop)` this function builds the EB levels

   and fills the implicit function :cpp:`MultiFab` (the later being used to

   construct the level-set function). Note that this also makes the particle EB

   levels point to the fluid eb levels.

3. Call :cpp:`mfix::build_eb_levels(gshop)` and

   :cpp:`mfix::build_particle_eb_levels(gshop_part)`. These functions build the

   EB levels, and fill the implicit function :cpp:`MultiFab` (the later being

   used to construct the level-set function).

4. (Optional, do this if you did 2.) Call

   :cpp:`mfix::build_particle_eb_levels(gshop_part)` **last**. This will update

   the particle EB levels.

MFiX's EB Data Structures

   //! EB levels representing fluid boundary conditions

   Vector<const EB2::Level *> eb_levels;

   //! EB levels representing particle boundary conditions (same as

   //! `mfix::eb_levels` but additional walls at MI BCs).

   //! `mfix::eb_levels` but might include additional walls at MI BCs).

   Vector<const EB2::Level *> particle_eb_levels;

   //! EB factory that lives on the fluid grids

   :cpp:`level_sets[lev]` on each level.

Special Cases Involving Level-Sets

----------------------------------

The level-set function is filled by the `mfix::fill_eb_levelsets()` function.

There are two special cases involving level-sets:

1. Mass-Inflow boundary conditions are not given EB walls. However, we don't

   want particles to fall out of a MI either, so at the very end of the

   `mfix::fill_eb_levelsets()` function we call `mfix::intersect_ls_walls()`.

   This performs an intersection operation with the level-set representing a

   wall at each MI.

2. Box geometries and regular geometries are comprised entirely out of planar

   surfaces. Therefore the levelset is not construction out of an EB factory (as

   would be the case for all other geometries). But out of an intersection with

   all planar surfaces. This has the advantage of correctly describing corners.

Fluid Reconstruction

--------------------