how to combine/concatenate catalogs?

Is there a way to combine catalogs? For example, let's say I have N cases:

```python
cat_list = []
for case, info in case_dict.items(): 
    b = Builder(**info)
    cat_list.append(b)

master_cat = ecgtools.concatenate(cat_list)
```

Possibly related to #15, though that seems somewhat out of date.