This version adds more documentation and does work behind the scenes to better organize the code and make writing extensions simpler. Refactor all unit tests to use pytest instead of unittest. Reorganize unit tests to be more consistent and to remove redundant fixtures. Modernize the build system using updated standards. Reorganize the core analyses to separate measures, transitions functions, and outputs functions. The high-level psifr.fr API remains the same, so this change only affects development of new analyses.