Implement Sensitivity Analysis

[okennedy]

Depends on #216 first.

Sensitivity (aka Influence) is a measure of how much of an impact a particular source of uncertainty has on a query result. In our context, it would be great to have a mechanism which, given a query and a specific variable identifier (aka a Reason), would compute the variance or entropy in the best guess result relative to the distribution of possible values assigned to the specific variable.

• http://dl.acm.org/citation.cfm?id=1989411
• http://dl.acm.org/citation.cfm?id=1880176