(NC-198) Add classification/inheritance-maxima.

This function, when given a map of groups to their ancestors, returns
those groups that are maxima according to the partial ordering defined
by inheritance (where a < b iff a is an ancestor of b). The returned
groups are not less than any other given group by this ordering, meaning
that they are unrelated to all other returned groups.
(NC-198) Add classification/conflicts.

Add a new schema, ClassificationConflict, for the description of
conflicts between classifications, which looks like a Classification
except that the values of all maps are sets of Classification values.

Add classification/conflicts, which takes classifications and returns
a ClassificationConflict map with the conflicts, or nil if there are no
conflicts. Also add some tests for this function.
WIP: need to create conflict details for error response