Make threads daemons
Add downsize filter
Test downsize filter
Add cache filter
Update make_sinograms

to send all the time and more than one tomograph.
Restructure coroutines tests
Add stall filter
Add stall test
Update backproject filter
Update backprojection algorithm

to work on the whole volume
Add ufo-based backprojection of a whole volume

also rename it to just Backproject and make it callable
Add PickSlice filter
Add PickSlice test
Fix docstrings and references
Restructure API reference
Fix problem with trailing s after ``
Drop num_images from average_images filter
Update average_images test
Add Acquisition to experiments base

Improves modularity by breaking the experiment into
smaller acquisition parts.
Update experiment base class