Include ancillary services functionality

[c-moeller]

@FHell @e-zolotarevskaya I created an issue in the same repository where we collect all the other open_plan issues to keep all our tasks in mind.

Maybe we can use this as an overview issue and link issues which describe this task in more detail to this. What do you think?

https://github.com/oemof/oemof-solph/issues/778