Introduce ProbabilityAnalysis class for quantitative analysis.

This ProbabilityAnalysis class performs what FaultTreeAnalysis class did
for probability analysis with minimal cut sets.
Add tests for ProbabilityAnalysis class.
Remove ProbOr and ProbAnd from FaultTreeAnalysis class.

This functions belong to the ProbabilityAnalysis class.
Refactor reporting probability analysis results and report number of
sets used.
fixes #55
Remove probability calculations from FaultTreeAnalysis class.
Create UncertaintyAnalysis class. Refactor functions from
FaultTreeAnalysis.