event has been renamed event_tag, to be consistent with the other tag-creating functions.

Memory-tag creating function memory_tag has been added to module history.

ColumnarFile has been moved to module history_medium, and tag-creating function columnarfile_tag has been moved to module history. Module columnarfile no longer exists. Thus, the four basic tag-creating functions (textfile_tag, event_tag, columnarfile_tag, and memory_tag) now require nothing more than the importing of module history.

Both text and columnar files contain method set_name and it can now be called anytime between records to start a new file (typically to prevent the creation of very large files).

Method history (name) has been added to both Tag and HistoryMedium. It returns the history stored under the given name, if supported by a particular medium. Otherwise it throws an exception. Currently only memory tags support this method.