Clean up edge cases in calculating share fraction in sim._raw_materials_to_finished_goods()

[LarrySnyder]

stockpyl/src/stockpyl/sim.py

Lines 856 to 865 in 26792d6

	# If units_ordered == 0, allocate into equal shares. (This can happen if the original
	# order was backordered, or if there is initial RM at the start of the simulation.)
	# TODO: allocate in a smarter way in this case?
	if units_ordered == 0:
	share_frac = {prod_index: 1 / len(prods_for_rm) for prod_index in prods_for_rm}
	else:
	# TODO: what if order_quantity_fg = 0 (e.g., because of backorders)?
	share_frac = {prod_index: node.state_vars[period - OLT - SLT].order_quantity_fg[prod_index] \
	* node.NBOM(product=prod_index, predecessor=None, raw_material=rm_index) \
	/ units_ordered for prod_index in prods_for_rm}

Allocate in a smarter way if units_ordered == 0?

Better handling for case where order_quantity_fg == 0 (e.g., because of backorders)?