Design Choice for how to show energy flow directions

[mstich0]

For not creating an extra issue: It would be great to have the possibility to start dragging a connection from the output AND from the input of any port (currently only from output is possible and it is very counterintuitive).

Now to the actual issue:
Currently we have outlows on the right side of the component which can be connected to the inflow on the left side of a component.
If a flow can happen in both directions this is not visible to the user. At the moment this is only the case for the DSO components, however there are good reasons to also implement a component like a transmission line from oemof into open plan to allow bidirctional flows if wanted.

We should therefore come to design that makes it visible to the user in which direction a flow goes or if it is bidirectional. One option is to include a small one/bi-directional arrow in the graph of the connection (if possible).

With this it would also be possible have a storage only be connected with one (bidirectional) flow to the bus (finally no more cross over for the storage)
The storage component would recognise if connected only with one bus and if so it would set input and output to this bus automatically.

also it would be nice to implement an option for converters to flip the side of inputs and outputs. This would be necessary once somebody wants to build some kind of circular energy system which would currently look not too appealing: