New feature: Draw simple processes

[fwitte]

It would be very benefitial, if the user could highlight specific simple processes and draw the respective lines into his diagram. Therefore I suggest the following addition:

• Add a calc_inidividual_isolines() or draw_individual_isolines() method.
• The user passes the property of the isoline(s) as well as inlet and outlet state (second property)
• The method draws the specified isoline into the diagram, coloring, linestyle, etc. can be passed similarly to the linespecs of the diagram lines.
• The lines are:
  • isenthalps
  • isobars
  • isochors
  • isentropics
  • isotherms
• The method needs to check
  • whether the input properties are a valid valuepair and
  • whether the starting point and the ending point are at different phases. If this is the case, the intersection with the saturation line(s) must be considered, too.