Add filter to select a grid

[gplssm]

Add the option to select one of many MV grids stored in the powerflow schema. Preferably based on the substation/mv grid district ID.

[gplssm]

Already done

• Added to ego.io table definitions
• Schema and tables newly created

Still to do

• implement filter in ego.powerflow
• test all this

[gplssm]

Seems to not be required anymore