Plugin subdirectories and cumulatives

Places plugin output in their own sub-TDirectories, and adds new generic classes for data that "accumulates" in SegmentSaver combinations. Breaks backwards API compatibility, but should still be able to read files produced by older pre-subdirectory SegmentSavers.

Plugin-based analysis

New "plugin" SegmentSaver architecture, plus miscellaneous Python and neutron decay event generator utilities.

Expanded and improved!

Significantly expanded from initial release, with useful code from other projects. Changes in file naming break backwards compatibility.

Initial version

Initial commit of code.