ISPN-3981 Enhance grouping API to support group manipulation methods

* First implementation. Added getGroup() and removeGroup() methods

This is the base implementation. Added methods to return and remove the keys from a group. This may be extended to support other functionality.

https://issues.jboss.org/browse/ISPN-3981
This would add IGNORE_RETURN_VALUES to the provided explicitFlags set if it was not empty, which we can't do.  Maybe instead we just create an EnumSet that starts with IGNORE_RETURN_VALUES and add explicitFlags to it and use the resulting EnumSet.