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)
Updated Javascript wrapper to use SI units

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
EES uses ~ as delimiter character

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Ancillary fitter fits and injects curves for all fluids

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
F factors included for all pairs in mixture excess parameters

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Added dhdp|T and reverted change to not allow input to equal output

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
More outputting in JSON for v5

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Merge branch 'master' of https://github.com/ibell/coolprop
Added melting curves notebook

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Added the ability to calculate Jacobian matrices using numerical derivatives

Signed-off-by: Ian bell <ian.h.bell@gmail.com>
Merge branch 'master' of https://github.com/ibell/coolprop
Fix bug with PS inputs caused by bad guess values; small changes to outputs for JSON for v5

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
small fixes to EES wrapper

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Merge branch 'master' of https://github.com/ibell/coolprop