Example custom merge test function

This is an example implementation of what I was suggesting in #7.

The default:

```
d = Dendrogram.compute(image)
```

is equivalent to 

```
d = Dendrogram.compute(image, merge_test_function=lambda n, **k: False)
```

and another extreme example is:

```
d = Dendrogram.compute(image, merge_test_function=lambda n, **k: True)
```

which will create a tree with no structures (i.e. 'always merge'). If we decide to go ahead with this, I will add tests.