Running call_compute_subroutines with the intel compiler using the following
FFLAGS: -g -O0 -init=snan,arrays crashed until I initialized several fields
below the sea floor.

Verified that using -finit-real=snan -ffpe-trap=invalid on the previous head of
development causes the call_compute_subroutines test to crash but the code on
the fix_uninit_mem branch (which eliminates unitialized values in the sinking
particle type below the sea floor) runs successfully.

It turns out that we need PON_remin=0, but none of the %remin(k) terms are used
if k > column_kmt. Introduced
marbl_interior_tendency_share_set_used_particle_terms_to_zero() to handle
setting sflux_out(k) and hflux_out(k) to 0. Next commit will add calls to the
same subroutine in ciso (which currently isn't tested for uninitialized memory)

Also added a comment to the else statement to make it clear which "if" it is
tied to

In the last commit I added calls to the new share routine to set hflux_out and
sflux_out below the sea floor for ciso sinking particles but I forgot to add
the additional dzr_loc = c0 line that is already in the non-ciso code and it
was caught by our CI scripts.

Testing on cheyenne with intel also showed that we need to intialize remin to 0
below kmt in marbl_interior_tendency_share_set_used_particle_terms_to_zero()