Porting the existing flex management functions into one agent

[Luc-Sol]

This first step is not only important to setup the structure for the EMS, but it will also clean up a lot of old and/or redundant code that is spread around amongst different GridConnection children.

[GillisHommen]

Is being picked up for batteries in: #174

Other types of flex should follow the same structure. Multiple flex-assets can also point to the same management-object, when 'integrated flex management' is needed.