Handle earmarked units in Policy.echelon_inventory_position()

[LarrySnyder]

Also check that predecessor/raw material are being handled correctly here:

stockpyl/src/stockpyl/policy.py

Lines 446 to 450 in 26792d6

	if self.type in ('EBS', 'BEBS'):
	# TODO: handle this case (for now, just setting pred_index and rm_index to None)
	# TODO: handle exclude_earmarked_units
	IP_before_demand = \
	self.node.state_vars_current.echelon_inventory_position(prod_index=product_index, predecessor_index=None, rm_index=None)