Allow users to implement their own distributions

FastRng/Distributions/Distribution.cs

@@ -16,7 +16,7 @@ public abstract class Distribution<TNum> : IDistribution<TNum> where TNum : IFlo
         this.fitter = new ShapeFitter<TNum>(this.ShapeFunction, rng, 100);
     }
 
-    private protected abstract TNum ShapeFunction(TNum x);
+    public abstract TNum ShapeFunction(TNum x);
         
     public TNum GetDistributedValue(CancellationToken token = default) => this.fitter.NextNumber(token);