Introduce a GDP pseudo-elasticity for freight transport activity

[khaeru]

Currently freight activity is projected to grow 1:1 with total GDP(PPP):

message-ix-models/message_ix_models/model/transport/demand.py

Lines 172 to 184 in a45fce4

	# Base year freight activity from file (n, t), with modes for the 't' dimension
	("fv:n-t:historical", "mul", exo.mode_share_freight, exo.activity_freight),
	# …indexed to base-year values
	(gdp_index, "index_to", gdp_ppp, literal("y"), "y0"),
	(fv + "0", "mul", "fv:n-t:historical", gdp_index),
	# Scenario-specific adjustment factor for freight activity
	("fv factor:n-t-y", "factor_fv", n, y, "config"),
	# Apply the adjustment factor
	(fv + "1", "mul", fv + "0", "fv factor:n-t-y"),
	# Select only the ROAD data. NB Do not drop so 't' labels can be used for 'c', next.
	((fv + "2", "select", fv + "1"), dict(indexers=dict(t=["RAIL", "ROAD"]))),
	# Relabel
	((fv_cny, "relabel2", fv + "2"), dict(new_dims={"c": "transport F {t}"})),

TODO introduce a 'fv-elasticity.csv' analogous to pdt-elasticity.csv, including:

• An input data flow.
• A placeholder file for the R12 node codelist.
• Adjustments to the calculation of fv_cny to incorporate this elasticity.

Discussed at the 2025-03-20 MESSAGEix-Transport meeting; for use in the LED scenario implementation in the ScenarioMIP/SSP project.

[khaeru]

Closed by #300.