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
* Internal draw_isolines method is used for plotting of the cycle. Thus, all invoked functions are extended to allow new parameters for plot line style and axis limits.
* Updated documentation and added example (rankine cycle)
* Enhanced handling of unit systems for plotting
* Fixed plotting issue caused by 9e832e76ff94b1d963e0049672348cee3137456c
* All tests in `test.plot.py` passes