Allow multiple outputs at OffsetConverter

[lensum]

For a project, I need to model a Converter with part-load depending efficiencies that outputs two flows. This is currently not possible with the OffsetConverter. I propose the following changes:

1. outputs should be allowed to contain multiple busses.
2. coefficients should expect a dictionary, where the keys are the output busses and the values are the tuples containing the necessary parameters.
3. The issues discussed in https://github.com/oemof/oemof-solph/issues/1010 should be fixed (misleading docu, usage guide and erroneous example).

[lensum]

I think the PR or the existing issue are better places for discussion (if necessary), therefore I'm closing this again.

[p-snft]

Thanks for bringing this up. IMHO, you should have both, a PR discussing the implementation and an issue discussing the general idea.

About the general idea: There are two possible options to implement something like this. Multiple inputs or multiple outputs. You cannot have both. So, wouldn't it be simpler to just use two OffsetConverters and one Converter? This allows the flexibility to do both.

[lensum]

If I understand you correctly, you mean that the Converter would effectively duplicate the incoming flow (by setting the conversion_factor to 1 for each output) and these are then fed into the OffsetConverter? That might work as well... It would also increase the number of variables, but maybe the pre-solve of efficient solvers would take care of that.

[p-snft]

You would have additional variables but code that is a lot easier to maintain. Also, it is a lot more flexible, e.g. allows to combine OffsetConverter and PicewiseLinearConverter.