Add some error checking in PartitionWoodFluxes

[billsacks]

After #1262 is resolved, I'd like to add some error checking to help prevent errors like this in the future. In particular, in the following code:

CTSM/src/biogeochem/CNProductsMod.F90

Lines 615 to 630 in 0f6985d

	if (pprod_tot > 0) then
	pprod10_frac = pprod10 / pprod_tot
	pprod100_frac = pprod100 / pprod_tot
	else
	! Avoid divide by 0
	pprod10_frac = 0._r8
	pprod100_frac = 0._r8
	end if
	! Note that the patch-level fluxes are expressed per unit gridcell area. So, to go
	! from patch-level fluxes to gridcell-level fluxes, we simply add up the various
	! patch contributions, without having to multiply by any area weightings.
	this%dwt_prod10_gain_grc(g) = this%dwt_prod10_gain_grc(g) + &
	dwt_wood_product_gain_patch(p) * pprod10_frac
	this%dwt_prod100_gain_grc(g) = this%dwt_prod100_gain_grc(g) + &
	dwt_wood_product_gain_patch(p) * pprod100_frac

I'd like to refactor this and add an assertion:

• Move the final block (which does the addition of fluxes) into the if
• Change the else block: don't bother setting frac (because now nothing will happen after that else block, with the above change); but instead, call endrun if dwt_wood_product_gain_patch(p) > 0

[billsacks]

Blocked: depends on #1262

[billsacks]

Since #1262 is fixed, this is unblocked.