DC Source Status, Command Additions

Submitted by Dimitri Butvinik, Xantrex/Lithionics.

New DGNs, DC_SOURCE_STATUS_11 and DC_SOURCE_COMMAND. Supports additional features of an "intelligent" battery management system. Also includes clarifications on terms used in related DGNs - Relative Capacity, State of Charge, State of Health, Capacity Remaining.