Generate: add `RandomNormalNumber()` to return a `double`

This works now:

```cs
double x = ScottPlot.Generate.RandomNumber();
```

and it would be nice if this worked:

```cs
double x = ScottPlot.Generate.RandomNormalNumber();
```

(with similar overloads)

Note that there is already `ScottPlot.Generate.RandomNormal()` which returns `double[]`