Add KF-ZrF4 salt to the fluid properties

We need to add the KF-ZrF4 salt properties to TACOCAT for certain molten salt reactors, please see the paper below for the specific properties.

See paper:

https://www.tandfonline.com/doi/abs/10.13182/NT12-A13598

Start by reading the linked paper for the KF-ZrF4 fluid properties, then locate TACOCAT's existing fluid-property definitions and related tests or entry points. Done means the salt properties are represented consistently with the project's other salts and the relevant calculations or tests pass.

Written by the indexing model from the issue text.