Docs

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• [x ] Fully documented, including whats-new.rst for all changes and api.rst for new API

Documentation for both #178 custom dependencies and #179 strict checking. Exposing them in update_processes and drop_processes can be another PR #183 .

[feefladder]

currently, there may be two issues:

1. the explanation at strict checking is maybe a bit too detailed?
2. the processes for visualizing do not coincide with the names that are given to them in some hidden python code used for displaying graphs - is this a problem?