Supporting duplicates in component list

[vegardjervell]

First of all: This is not a major issue, but (at most) a minor annoyance that takes 5 lines of code to work around. If it requires more than very little time to fix, it's probably not worth it.

With that said:
At times I find myself wanting to model e.g. a LJ-mixture with some sigma-ratio or similar. Until now, I have solved this by for example initialising saftvrmie with two components that are fairly similar (e.g. AR/KR) and then using set_pure_fluid_param() and get_pure_fluid_param() to change the fluid parameters. However, it would be nice to be able to just initialise with LJF/LJF, and then set_pure_fluid_param(), thus saving the time spent extracting parameters for one component, scaling them, and then setting parameters for the other component.

I don't know exactly how the parameter sets are handled on the fortran-side, so this might be non-trivial to support, I'm just throwing it out as a suggestion.