Ipyflow

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* Always get freshest variable references during analysis and fix resulting reactivity bugs;

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* Bugfix for restart / run all;
* Bugfix for IPython version
* Unify dependency management for statements and cells;
* Bugfix for correct active cell use during nbextension active cell run;
* Fix IPython dependency for different Python versions;
* Allow exec schedule recomputation on cell content change;
* Add API for symbol tagging;
* Code HTML widget with links for jumping to cells / copying slice;
* Misc other bugfixes;

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* Allow for loading / unloading as an ipython extension;

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* Bugfix for nbextension active cell notification;

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* Bugfix for kernel change state transition logic in nbextension

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* Even better behavior around dependency tracking between cells;
* Bring nbextension to parity with labextension mostly;
* Misc stability improvements around kernel switching / restarting;