Set lower limits to avoid NaNs. More fit testing.

```dpln``` still not fitting properly.
```pln``` largely works. The testing data has too little of a power-law tail to lock on well. More testing is needed. The moments are a bit tricky and are not yet working. Actually they don't exist if the order is greater than alpha!

```pln``` is (I think) properly normalized in its form, since it can be fitted as mixed model of the two limiting cases (alpha, beta -> inf) (see Reed, top of 1744).