typos from MR114 (!134)

1. Construct an implicit function representing the geometry.

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

2. Call :cpp:`mfix::build_eb_levels(gshop)`. This function builds the EB levels.

   Note that this also makes the particle EB levels point to the fluid eb levels.

3. Call ``mfix.fill_eb_levelsets``. This function fills the level-set ``MultiFab``.

   way the level-set always has the appropriate resolution to resolve structures

   in the EB, even if the fluid is defined on a fairly coarse grid.

2. MFIX-Exa is running in multi-level mode (:cpp:`nlev > 1`). The the parameter

2. MFIX-Exa is running in multi-level mode (:cpp:`nlev > 1`). The parameter

   :cpp:`mfix::levelset__refinement` is ignored. :cpp:`mfix::level_sets` then

   follows the rest of MFIX-Exa, i.e. it is defined on the particle grids on all

   levels.

- Sort the particles on each grid by bin, using a parallel counting sort. We use Thrust's `exclusive\_scan` function to implement the prefix sum phase of the sort, and hand-coded kernels for the rest. This step does not actually involving rearranging the particle data - rather, we compute a permutation that would put the particles in order without actually reordering them.

- Once the particles are sorted by bin, we can loop over the particles in neighboring bins. We make two passes over the particles. First, we launch a kernel to count the number of collision partners for each particle.

- Then, we sum these numbers and allocate space for our neighbor list.

- Finally, we make a another pass over the particles, putting them into to list at the appropriate place.

- Finally, we make another pass over the particles, putting them into the list at the appropriate place.

Note that we build a full neighbor list, meaning that if particle `i` appears in particle `j`'s list, then particle `j` also appears in particle `i`'s list. This simplifies the force-computation step when using these lists, since the forces and torques for a given particle can be updated without atomics.

Once the neighbor list has been constructed, collisions with both particles and walls can easily be processed.

MFIX-Exa currently runs on both CPU-only and hybrid CPU/GPU architectures.