Probability computation for fault trees has rounding error due to use of Double

[reteprelief]

We should use BigDecimal to deal with rounding errors.
See #1643 as related issue.

[reteprelief]

See #1651 as an issue to be addressed to fully satisfy the ability to calculate the occurrence probability correctly. Currently the property does not return the exact user supplied value, thus, the BigDecimal based computation still produces imprecise values - at least the computation does not introduce additional precision error.