Propane convergence close to critical point

[thorade]

For pure fluid propane, I think I see convergence issues close to the critical point. In the GUI, select propane, then enter

1. first row "crt"
2. second row density 219V or density 219 and quality 1
3. third row density 221L or density 221 and quality 0

Third row will give an error, but different one for the ways of specifying the same.

DLL version 10.0, even though I think I had a newer one in between.

[ianhbell]

Yeah, that's always tricky to do those calculations because the slope of the function to be solved goes infinite. The only super robust solution is to use the superancillary equations: https://pubs.aip.org/aip/jpr/article/53/1/013102/3270194