[fluidsystem][brineco2] fix fugacity coeff

Merged Beatrix Becker requested to merge fix/fugacity-coeff-in-brine-co2 into master

Add ratio of pressures to calculation of fugacity coefficients. Since, e.g., for a w-component in the liquid phase:

f_l^w = f_g^w
\varphi_l^w x_l^w p_l = \varphi_g^w x_g^w p_g
\rightarrow \, \varphi_l^w = \varphi_g^w  \frac{x_g^w}{x_l^w}\frac{p_g}{p_l} = \frac{x_g^w}{x_l^w}\frac{p_g}{p_l}

This might not have a very strong effect because pressures may not deviate much. There is no test for that, though. The only test where brineco2 is used, as far as I can see, are the co2-model tests. They do not use the fugacity coefficients at all.

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