Add material/product stocks, demand, and outflow variables

[MarianneZZotin]

Summary

Adds a materials codelist covering material flow analysis (MFA) variables
developed during the CIRCOMOD project. These describe
material and product stocks, demand, and outflows across the
buildings, vehicles, electricity and transport infrastructure sectors.
These were originally defined in the project workflow repository (iiasa/circomod-workflow)
and are proposed here for general use.

What's included

Added under definitions/variable/materials/ as a self-contained subfolder,
so no existing files are modified.

Stocks

• material-stock.yaml: in-use material stocks
• product-stock.yaml: in-use product stocks

Demand

• final_material_demand.yaml: final material demand
• final-product-demand.yaml: final product demand

Outflows and losses

• material-outflow.yaml: material leaving the in-use stock
• stock-retirement.yaml: retirement of products from stock
• material-losses.yaml: losses along the material cycle

Tags (used to expand the variables above across sector and product dimensions)

• tag_demand_sector.yaml, tag_demand_sector_disaggregated.yaml
• tag_building_types.yaml
• tag_vehicles_types.yaml,
• tag_transp_infra_types.yaml
• tag_engmaterial.yaml
• tag_electricity_types.yaml

A diagram of the underlying MFA system (CIRCOMOD_MFA_System.png) is included
to show how the variables map onto a common material flow system.