Report CAP, CAP_NEW, CAP_RET for vehicles

[khaeru]

This is needed for DIGSY scenarios (cf. #373, #400).

It is distinct from #296, which is for the CircEUlar project. There, the focus is on integration of MESSAGEix-Transport with MESSAGEix-Materials/ the MESSAGE 'cap_comm' formulation.

Here, we instead need to:

• Extend/improve IAMC-structured reporting output or add new, dedicated output data flows (similar to .transport.base) that include numbers of vehicles added and scrapped (@khaeru)
• Multiply these by aggregate/estimated material intensities (@r-aneeque or/and @khaeru)
• Provide these to @macflo8 for use in other DIGSY scenarios.