A connivence overload for rBeta

The numpy version of the beta distribution takes two extra parameters `loc` and `shape`. In order to replicate this, I have this function:

```
auto randomBeta(double alpha, double beta, double location, double shape) {
    return (rBeta(alpha, beta) * shape) + location;
}
```

It would be nice to have something like this in by default.
