Fixed lattice cell overlap error

The error was caused by particles that were in a lattice cell with surfaces close to the origin but were outside of the lattice bounds (both hex and rect lattices).

src/geometry.F90

@@ -26,6 +26,8 @@ contains
     type(Particle), intent(inout) :: p
     logical                       :: in_cell
 
+    integer :: lat_ind         ! index in lattices array
+    integer :: i_xyz(3)        ! lattice indices
     integer :: i               ! index of surfaces in cell
     integer :: i_surface       ! index in surfaces array (with sign)
     logical :: specified_sense ! specified sense of surface in list
@@ -33,6 +35,27 @@ contains
     type(Surface), pointer, save :: s => null()
 !$omp threadprivate(s)
 
+    ! We need to check and make sure the particle is not outside of a lattice
+    ! before checking surfaces. The lowest coordinate level for a particle that
+    ! is outside of a lattice should be an infinite universe.  Unfortunately,
+    ! it's not.  The following code is a bit of a hack to avoid that fact.
+    if (p % coord % cell /= 0) then
+      ! ^ New particles will have p % coord % cell = 0.
+      if (cells(p % coord % cell) % type == CELL_LATTICE) then
+        ! The particle is in a lattice cell.
+        lat_ind = cells(p % coord % cell) % fill
+        i_xyz = (/p % coord % lattice_x, p % coord % lattice_y, &
+             &p % coord % lattice_z/)
+        if (.not. lattices(lat_ind) % obj % are_valid_indices(i_xyz)) then
+          ! The particle is outside of the lattice.  There shouldn't be surfaces
+          ! outside the lattice (on the lowest coordinate level) so we can
+          ! simply check for cell equivalence.
+          in_cell = associated(c, cells(p % coord % cell))
+          return
+        end if
+      end if
+    end if
+
     SURFACE_LOOP: do i = 1, c % n_surfaces
       ! Lookup surface
       i_surface = c % surfaces(i)