Fixes issue #77

As per discussion in issue #77 with Dave K., this commit adds delta/2
to the requested `endtime` to ensure that starttime and endtime are
fully inclusive in Edge-related I/O factories. I would have liked to
generalized this and reduce the duplicate code between EdgeFactory
and MiniSeedFactory, but couldn't figure out how to do this without
adding considerably *more* code to the project than what is in this
commit.