mortonne/psifr: Psifr 0.5.0

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.