fix operation order error in _spectral_helper and implement more tests to catch these sorts of problems in the future
Make backend_pgf more flexible when saving to file-handles or streams.
Merge pull request #2571 from toddrjen/specfix

This fixes thee probllem brought up in the mailing list with the recent spectrum improvements
user guide restructure
Merge pull request #2554 from marqh/userGuideShape

User Guide Structure
Merge pull request #2587 from pwuertz/fix_1625

Make backend_pgf more flexible when saving to file-handles or streams (fix #1625).