Add sulfur into VESIcal

[simonwmatthews]

I think a really useful advance would be to incorporate sulfur models into VESIcal. This is a bit more complex than adding new H2O or CO2 models, so will need quite a bit of attention. I also know there is quite a bit of interest in this from the community and so I thought it would be good to flag it as a future feature we are thinking about.

The steps to doing this are:

1. Code up a sulfur solubility model (e.g., https://doi.org/10.1016/j.gca.2022.08.032).
2. Code up a sulfur fugacity model (ideally incorporating H2O and CO2, if such a model exists).
3. Modify the underlying VESIcal methods to cope with a third volatile phase. This could be a real nightmare, or might be surprisingly straightforward. VESIcal was constructed with an expansion like this in mind, but some of the numerical methods may have been optimised specifically for two components.

I am unlikely to have time to do much work on this myself, but I am very happy to help someone else do this!

[kaylai]

Commenting here as I am thinking about this a lot right now. I am working to go through the code for Sulfur_X and planning to add this into VESIcal. Eventually, I would like the addition of a handful of sulfur models to be a key component of "VESIcal 2.0", which would be a new publication with a whole host of features.