`EmInput` should not be connected to the grid

[sebastian-peter]

EmAgents in simona are not connected to the grid, but solely control the power in/output of system participants. Thus, EmInput should not have a node attribute, i.e. should be derived from AssetInput, not SystemParticipantInput.

Documenation also needs to be adapted accordingly: Move the em result page to Output instead of Participant Related Models (one layer up). Also, em input documentation needs to be created in the first place.

Edit: Maybe keeping em results in place is better.

cc @t-ober @danielfeismann

[sebastian-peter]

Implementation has been done in #959, changes in documentation still have to be made