#13 Fix speed problems from FilteredEntryGroup.FeatureEnumerator

[Kim Rutherford]

FilteredEntryGroup.FeatureEnumerator is too slow. The
contructor should call getFilteredFeatures() and remember
the results.

hasMoreFeatures() and nextFeature() should throw an
Exception if the EntryGroup changes. java 1.2 calls that
"fail-fast" and throws a ConcurrentModificationException.