fix incorrect derivative of rock compressibility w.r.t. pressure

since

f(x) = 1 + 0.5*g(x)*g(x)

the derivative is

f'(x) = 0 + 2*0.5*g(x) * g'(x) = g(x)*g'(x)

note that the previous incorrect values do not affect the quality of
the obtained results (if the tolerance of the non-linear solver is
chosen to be small enough), but it may have deteriorated convergence
rates.

opm/core/props/rock/RockCompressibility.cpp

@@ -101,7 +101,8 @@ namespace Opm
         if (p_.empty()) {
             // Approximating poro multiplier with a quadratic curve,
             // we must use its derivative.
-            return rock_comp_ + 2 * rock_comp_ * rock_comp_ * (pressure - pref_);
+            const double cpnorm = rock_comp_*(pressure - pref_);
+            return rock_comp_ + cpnorm*rock_comp_;
         } else {
             return Opm::linearInterpolationDerivative(p_, poromult_, pressure);
         }