Check brine fluidsystem

The binary diffusion coefficient of NaCl seems to be wrong. Also there is no source available for the implemented one. Because of that the implemented parameters should be checked and sources sould be added. Further there are relations e.g. for density where the source is not mentioned in the comments. Also there are some todos left to do which could be essential.

• correct diffusion coefficient
• check parameters and add sources
• improve comments and add sources for relationships
• todo check contribution of NaCl on thermal conductivity
• todo find better description for calculation of the isobaric heat capacity
• check solid heat capacity of NaCl which is given in J/mol K
• check fugacity coefficient in brine and brineair
• update reference solutions