Address more @smharper comments

src/api.F90

@@ -169,7 +169,7 @@ function openmc_cell_set_temperature(index, T, instance) result(err) bind(C)
           end if
         end associate
 
-        ! assign error codes for outside of temperature bounds provided the
+        ! Assign error codes for outside of temperature bounds provided the
         ! temperature was changed correctly. This needs to be done after
         ! changing the temperature based on the logical structure above.
         if (err == 0) then
@@ -627,7 +627,7 @@ function openmc_material_set_densities(index, n, name, density) result(err) bind
     real(C_DOUBLE), intent(in) :: density(n)
     integer(C_INT) :: err
 
-    integer :: i, j, k
+    integer :: i
     character(C_CHAR), pointer :: string(:)
     character(len=:, kind=C_CHAR), allocatable :: name_
 
@@ -868,6 +868,11 @@ function openmc_tally_set_nuclides(index, n, nuclides) result(err) bind(C)
     end if
   end function openmc_tally_set_nuclides
 
+!===============================================================================
+! TO_F_STRING takes a null-terminated array of C chars and turns it into a
+! deferred-length character string. Yay Fortran 2003!
+!===============================================================================
+
   function to_f_string(c_string) result(f_string)
     character(kind=C_CHAR), intent(in) :: c_string(*)
     character(:), allocatable :: f_string

src/simulation.F90

@@ -394,9 +394,9 @@ end subroutine initialize_simulation
 
   subroutine finalize_simulation()
 
-    integer :: i       ! loop index for tallies
-    integer :: n       ! size of arrays
 #ifdef MPI
+    integer    :: i       ! loop index for tallies
+    integer    :: n       ! size of arrays
     integer    :: mpi_err  ! MPI error code
     integer(8) :: temp
     real(8)    :: tempr(3) ! temporary array for communication