Symbolic regression variable coefficients are never mutated using an additive random constant

[gkronber]

Bug in the code:

      // 50% additive & 50% multiplicative
      if (random.NextDouble() < 0) {
        double x = NormalDistributedRandom.NextDouble(random, Symbol.WeightManipulatorMu, Symbol.WeightManipulatorSigma);
        weight = weight + x * shakingFactor;
      } else {
        double x = NormalDistributedRandom.NextDouble(random, 1.0, Symbol.MultiplicativeWeightManipulatorSigma);
        weight = weight * x;
      }