Add aggregation examples

[bdice]

We could add more examples for some or all of the following:

• Group by state point keys: The aggregates are grouped by multiple state point keys.
• Group by arbitrary key function: The aggregates are grouped by keys determined by a key-function that expects an instance of :class:~.signac.contrib.job.Job and return the grouping key.
• Using a completely custom aggregator function when even greater flexibility is needed.
• Using sorting/selection in conjunction with other aggregator parameters.

Originally posted by @bdice in #122 (comment)