Unwrapper handling of empty entries within a batch

[francois-drielsma]

Two things I observed:

• With batch_size=1 and an empty particle_graph, the unwrapper crashes when buidling the unwrap_map
• With batch_size=2 and a particle_graph with only one batch_id, the unwrapper returns a single entry for that data product

It looks to me that if a data product, say particle_graph, has no corresponding row for a particular batch ID, the number of np.arrays returned by the unwrapper may be different from other data products that do, which is undesirable. If the batch_size is 16, we should have 16 objects per data product returned, even if they are empty. One way to do this is to use the batch_idx_max to resize outputs instead of appending them. Correct me if I am misreading the code.