Forthcoming fixes

Changes for upcoming 7.6.6

1. Made table counting classes for the chi-square and g-square tests immutable to be safe in multi-threaded environments in response to a worry discovered when moving to JDK 21.
2. Made some updates to the implementation revision of the final FCI orientation rules in response to a review from Peter Spirtes.
3. Revised implementation of the AD Tree class and incorporated it into the Chi-Square and G-Square discrete independence tests as an alternative to sample counting.
4. Revised DiscreteBicScore to ignore unattested categories for variables.