Missing process step in state diagrams of heat pumps using flash tanks

[jfreissmann]

The model classes HeatPumpFlash and HeatPumpFlashTrans miss the desuperheating process step of the compressed gas exiting the first compressor stage. This issue stems from the fact, that the get_plotting_data method of the Drum component (used within these heat pumps as a flash tank) only return a line from the saturated liquid outlet to the saturated vapor outlet.

I created a Pull Request (see: oemof/tespy#521) to fix this issue by adding more process lines to be returned from the get_plotting_data method, so that the state diagrams can be fixed. Therefore the issue persists until the PR is merged and we bump the tespy version. This means we should prioritize issue #2 and get the tespy models up to date.