Added easy plotting of cycles (closes #174)

I have added a new method to PropsPlot  class, to allow easys plotting of processes and cycles from a list of State objects as described in #174.

* new draw_process method
* updated documentation and added example (rankine cycle)
* enhanced handling of unit systems for plotting
* fixed plotting issue caused by 9e832e76ff94b1d963e0049672348cee3137456c