ToDo: Get signed phase currents from all gridmeters

[premultiply]

All (grid) meter templates have to be checked and enhanced to get signed phase currents where possible.

Requirement for load management https://github.com/evcc-io/evcc/pull/3453

Positive sign for import on phase, negative sign for export.

Signed values can be made by mul(phasecurrent, sign(phasepower)) where they are not natively signed.