Add constants for usecase scenarios

[andig]

I've just caught myself using the wrong scenario id for active power MCP vs MGCP (used 1 in both cases):

• MGCP §1 (UC TS):
  • S1 = Total Power Factor (cos φ)
  • S2 = Total Active Power per phase + total
  • S3 = Total Feed-in Energy (export)
  • S4 = Total Consumed Energy (import)
  • S5 = Phase-specific currents
  • S6 = Phase-specific voltages
  • S7 = Frequency
• MPC §1 (UC TS):
  • S1 = Total Active Power (per phase + total)
  • S2 = Total Energy Consumed (import)
  • S3 = Phase-specific currents
  • S4 = Phase-specific voltages

This has been discussed before and declined since the standard does not provide these named constants. However, I'd still find it valid to do so as part of the use case definition. Prior art: https://github.com/evcc-io/evcc/pull/29701/changes#diff-6bbf17b679bd8a2e4951d775614aa100b81562e1bff586ad83f68b7d5dc7f02c