Fixed plotting issue caused by changed error handling in PropsSI

- Same output as input variable not allowed in Props and PropsSI anymore
Added possibility to plot isolines for x- and y-values
Added possibility to override default line styles for isolines

- Isolines will be plotted with default lines styles.
- This can now be overridden in PropsPlot.draw_isolines with optional argument
Allow axis limits for plotting of isolines

- added optional axis_limits parameter to PropsPlot.draw_isolines
Added method for easy plotting of processes and cycles

- updated PropsPlot documentation
Enhanced handling for unit systems in PropsPlot

- improved switching of unit systems (kSI and SI) for plots
- added proper axis labels for SI unit system
- updated docstring comments
Merge pull request #195 from spinnau/plot_process

Added easy plotting of cycles (closes #174)