Distributions: increase Brent root-finder accuracy in Beta, F and Student-T inverse CDF to adapt to fixed Brent behavior

src/Numerics/Distributions/Beta.cs

@@ -653,7 +653,7 @@ namespace MathNet.Numerics.Distributions
                 throw new ArgumentException(Resources.InvalidDistributionParameters);
             }
 
-            return Brent.FindRoot(x => SpecialFunctions.BetaRegularized(a, b, x) - p, 0.0, 1.0, accuracy: 1e-8);
+            return Brent.FindRoot(x => SpecialFunctions.BetaRegularized(a, b, x) - p, 0.0, 1.0, accuracy: 1e-12);
         }
 
         /// <summary>

src/Numerics/Distributions/FisherSnedecor.cs

@@ -406,7 +406,7 @@ namespace MathNet.Numerics.Distributions
 
             return Brent.FindRoot(
                 x => SpecialFunctions.BetaRegularized(d1/2.0, d2/2.0, d1*x/(d1*x + d2)) - p,
-                0, 1000, accuracy: 1e-8);
+                0, 1000, accuracy: 1e-12);
         }
 
         /// <summary>

src/Numerics/Distributions/StudentT.cs

@@ -510,7 +510,7 @@ namespace MathNet.Numerics.Distributions
                 var h = freedom/(freedom + (k*k));
                 var ib = 0.5*SpecialFunctions.BetaRegularized(freedom/2.0, 0.5, h);
                 return x <= location ? ib - p : 1.0 - ib - p;
-            }, -800, 800, accuracy: 1e-8);
+            }, -800, 800, accuracy: 1e-12);
         }
 
         /// <summary>