Add `.model.transport.material`

[khaeru]

This module will prepare data for MESSAGEix-Materials/the MESSAGE 'cap-comm' representation, based on input files from #296.

This includes:

• Prepare dummy/template input data files/flows for:
  • Current values and projections of vehicle class shares, for LDVs and non-LDVs.
  • Data files related to material intensities.
• Read these files and compute values for {in,out}put_cap{,_new,_ret}.
• Test and ensure correct units, consistent with the base .model.material representation.