Move methods from TXPipe base stage and update documentation

[joezuntz]

The largest changes here are to expand the docstrings of the PipelineStage class, which will soon be API-documented on the TXPipe doc pages.

In addition, we upstream three methods from TXPipe, which currently adds them to the base stage:

• time_stamp - prints a time stamp with a tag to stdout
• memory_report - prints a description of the memory available on the system
• combined_iterators - combines together multiple iterate_hdf5 calls for matching files / groups.