v0.4.0 Importance etc. (2025-05-20)

• Implemented importance output
• Implemented beta and gamma distributions
• Implemented fault tree property computational_order (1 for rare approximation)
• Renamed tolerance to computational_tolerance
• Fixed partial sums should be initialised as float zero, not integer zero
• Converted model_type from string to enum
• Omitted/abbreviated irrelevant attributes in object representations