Clarify suggested density slope in documentation is for TDS

[JoerivanEngelen]

@tessodk, pointed out to me that we suggest a value of 0.7134 for the density slope for the VDF in the WQ section of the documentation. However, we do not mention that this is for TDS g/l.

@tessodk observed values used of 1.316 and 1.34 for different models using CL g/l.

Would be ideal to suggest an industry standard value for chloride as well. Though the uncertainty introduced by these density slopes is not huge compared to other uncertainties in groundwater models, see:

Kohfahl, C., Post, V. E. A., Hamann, E., Prommer, H., & Simmons, C. T. (2015). Validity and slopes of the linear equation of state for natural brines in salt lake systems. Journal of Hydrology, 523, 190–195. https://doi.org/10.1016/j.jhydrol.2015.01.054

Requirements

• Clarify the suggested value for the density slope is for TDS g/l in the documentation
• Supply value for chloride as well
• Also suggest values in MODFLOW6 part of the documentation