[brineair] reuse brine fluid system and fix liquid density and nacl fugacity

This makes use of the introduced fluid state adapter for mapping to the right
phase and component indices.

This fixes:
- calculation of the liquid mixture density (for the case of it being composition-depent)
- make fugacity coefficient of NaCl zero (we assume NaCl to stay in the
water phase anyway throughout this fluid system)
- adapt reference solution