Added __iadd__ to PPSD object

This adds `__iadd__` functionality to the `PPSD` object.

This allows for conveniently combining e.g. daily PPSDs to weekly/monthly/yearly PPSDs.:

```python
week42 = ... # create empty ppsd object

for ppsd in [mon, tue, wed, thu, fri, sat, sun]:
   week42 += ppsd
```

A few things:
 * This is more or less blocked by #896.
 * I am looking for opinions. If you intend to merge this, I can provide tests and extend the tutorial.
 * If the `PPSD` object had a `copy()` method, this could easily be used to provide an `__add__` method.

Indeed, this might be an unwanted surprise. Can't we just replace the `hist_stack` attribute with `other.hist_stack` (and update the `times_used`)?