*Switched remap_via_sub_cells() to use "new" algorithm

- A newly added unit test revealed that remapping was not conserving for
  truly vanished layers at the bottom of the model.
  - Discovered/reported in #472 by @ashao.
- A static logical parameter "old_algorithm=.true." was added in
  d43d8fd to keep using the old algorithm when answers changed
  unexpectedly. The new unit test explains why ansers changed
  and indicate that the new algorithm appears to be correct after all.
- Closes #472.

src/ALE/MOM_remapping.F90

@@ -76,7 +76,7 @@ module MOM_remapping
                                       !! changing the numerical result, except where
                                       !! a division by zero would otherwise occur.
 
-logical, parameter :: old_algorithm = .true.  !< Use the old "broken" algorithm.
+logical, parameter :: old_algorithm = .false. !< Use the old "broken" algorithm.
                                               !! This is a temporary measure to assist
                                               !! debugging until we delete the old algorithm.
 
@@ -1724,6 +1724,13 @@ logical function remapping_unit_tests(verbose)
   remapping_unit_tests = remapping_unit_tests .or. test_answer(v, 5, ppoly0_coefficients(:,2), (/0.,6.,0.,0.,0./), 'Limits PPM: P1')
   remapping_unit_tests = remapping_unit_tests .or. test_answer(v, 5, ppoly0_coefficients(:,3), (/0.,-3.,3.,0.,0./), 'Limits PPM: P2')
 
+  call PLM_reconstruction(4, (/0.,1.,1.,0./), (/5.,4.,2.,1./), ppoly0_E(1:4,:), ppoly0_coefficients(1:4,:) )
+  remapping_unit_tests = remapping_unit_tests .or. test_answer(v, 4, ppoly0_E(1:4,1), (/5.,5.,3.,1./), 'PPM: left edges h=0110')
+  remapping_unit_tests = remapping_unit_tests .or. test_answer(v, 4, ppoly0_E(1:4,2), (/5.,3.,1.,1./), 'PPM: right edges h=0110')
+  call remap_via_sub_cells( 4, (/0.,1.,1.,0./), (/5.,4.,2.,1./), ppoly0_E(1:4,:), ppoly0_coefficients(1:4,:), &
+                            2, (/1.,1./), INTEGRATION_PLM, .false., u2, err )
+  remapping_unit_tests = remapping_unit_tests .or. test_answer(v, 2, u2, (/4.,2./), 'PLM: remapped  h=0110->h=11')
+
   deallocate(ppoly0_E, ppoly0_S, ppoly0_coefficients)
 
   if (.not. remapping_unit_tests) write(*,*) 'Pass'