Added ProcessEventsWithoutTraces and docstrings

You can now process events without downloading the traces.  Also added a
test script.
Improved working with coincidences

Added a lot of docstrings, restructured the Coincidences class a bit and
added a test script.  Also, created the ProcessIndexedEventsWithoutTraces
class for use with Coincidences.
Tutorial: documented searching for coincidences

Cleaned up the code and docstrings a bit.
Merge branch 'tutorial'