Clean up notebooks

[cpaniaguam]

The notebooks directory could benefit from some purging. Some of them don't run with current versions of the package.

[cpaniaguam]

Some are not annotated:
notebooks/essential_tests.ipynb
notebooks/pyddm_tests.ipynb
notebooks/simplest_tut.ipynb

Others seem to test functionality -- perhaps they can be deleted? We now have a test suite.
notebooks/test_*.ipynb

[AlexanderFengler]

yes some spring-clean on this front definitely makes sense. Rule should basically be, if it's not useful as an independent tutorial (in which case it should be elevated to be part of the docs), then we should just kill it.