vert_dist_top veg check mishandles (?) some PFT types

[samsrabin]

As of ctsm5.4.047, this block does not explicitly handle nbrdlf_dcd_tmp_shrub or nc4_grass:

CTSM/src/biogeochem/CNFireEmissionsMod.F90

Lines 356 to 383 in 7409d32

	! Bare soil, won't be used
	if ( veg_type == noveg ) then
	ztop = nan
	! temperate and boreal evergreen needleleaf trees
	else if ( veg_type == ndllf_evr_tmp_tree .or. veg_type == ndllf_evr_brl_tree ) then
	ztop = 4.e3_r8 ! m
	! temperate and boreal trees
	else if ( veg_type == ndllf_dcd_brl_tree .or. veg_type == nbrdlf_evr_tmp_tree .or. &
	veg_type == nbrdlf_dcd_tmp_tree .or. veg_type == nbrdlf_dcd_brl_tree ) then
	ztop = 3.e3_r8 ! m
	! tropical broadleaf trees (evergreen and decidious)
	else if ( veg_type == nbrdlf_evr_trp_tree .or. veg_type == nbrdlf_dcd_trp_tree ) then
	ztop = 2.5e3_r8 ! m
	! shrubs
	else if ( veg_type >= nbrdlf_evr_shrub .and. veg_type <= nbrdlf_dcd_brl_shrub ) then
	ztop = 2.e3_r8 ! m
	! grasses
	else if ( veg_type >= nc3_arctic_grass .and. veg_type <= nc3_nonarctic_grass ) then
	ztop = 1.e3_r8 ! m
	! generic unmanaged crops
	else if ( veg_type == nc3crop .or. veg_type <= nc3irrig ) then
	ztop = 1.e3_r8 ! m
	! Prognostic crops
	else if (is_prognostic_crop(veg_type)) then
	ztop = 1.e3_r8 ! m
	else
	call endrun('ERROR:: undefined veg_type' )
	end if

The result is that they both fall into else if (veg_type == nc3crop .or. veg_type <= nc3irrig) and thus get ztop set to 0.001.

This is messy but has no impact for nc4_grass, as the other grasses do get 0.001. However, that's not the value for the explicitly-handled shrubs (0.002), so this is probably buggy behavior for nbrdlf_dcd_tmp_shrub.

Note: After #4112 merges to master, those WILL be handled explicitly, including the presumably buggy behavior for nbrdlf_dcd_tmp_shrub.

Worth noting: This kind of bug will not be possible once we resolve #3388!