Units with negative outputs
Currently, it is possible to create units with negative outputs (e.g. a cooling unit with negative maxGen), as mentioned in #76 (closed). The documentation of how to give the efficiency and operational point parameters of these units is missing.
I would actually like to avoid negative values for maxGen and instead model those as positive maxCons. We could add a 'considerInConversion' (three choices: 1-no, 2-asInput, 3-asOutput) in p_gnu that would also replace 'doNotOutput' (see issue #109 (closed)), or something similar. Arbitrary value mappings are probably not what we want, so maybe a set would be better (created using Spine Toolbox 'method'?). The conversion equations could then use (derived?) sets like gnu_input_as_input, gnu_output_as_input, gnu_output_as_output, gnu_input_as_output when determining whether v_gen should be multiplied with -1 and/or with slope.