Correct the absorptivity docstring

The terminology was confusing and also the math markup didn't
work.
Update coroutine docs to be readable and complete

Some docstrings were missing and there was the
foo(*args, **kwargs) problem because of the coroutine
wrapper. Also moved the consumer to the last position
by filters.
Split coroutine docs into user and devel part
Restructure data processing docs
Fix coroutine import in uca cameras
Remove unused is_async
Rename future.wait to the more appropriate .join

This will also help to provide a common Greenlet/Future interface.
Add gevent-based async decorator
Test kill feature of gevent
Test gevent on Travis
Use concert.config to disable run-time features
Remove multiprocessing for needle computation
Move asynchronous tools to concert.async
Move coroutine things to concert.coroutines[.base]
Implement complete Future interface