Batches from Python generator objects

Generating batches from Python generator objects would allow users to use complex and custom filtering in Python to create a high-performance batch object as a result.

This couples well with the new `iterate()` method, which reuses Python-side objects. This currently doesn't work without the copy:

```
from copy import copy
# Python generator object
selected_load_objs = (copy(load) for load in altdss.Load.iterate() if predicate_function(load))
selected_load_batch = altdss.Load.batch(objs=selected_load_objs)
```

The idea is to allow using the temporary iterator objects directly, since they are lightweight.