Opaque MetaModules + single-image Sequencer

Several major changes were made in this release.

Image Sequencer now stores only a single image, rather than multiple images, dramatically simiplifying the model: #824 (this is a breaking change)

Meta Modules were also refactored completely so as to become opaque - they remain self-contained modules whose inner steps only interact with the outer Sequencer through the mapping function passed at their initialization: #779, #764

See additional documentation in the README and thanks to our amazing and growing team of contributors; over 18 people have contributed in the past month, and over 61 overall:

https://code.publiclab.org/#r=image-sequencer

https://github.com/publiclab/image-sequencer/graphs/contributors