add ability to delete a composite factor

Add ability to delete a composite factor. Here is my proposal:

Add a concept of an emptyGroup in the grouper.properties. Sites should configure a location for this, e.g. next to the wheel group. e.g. etc:emptyGroup. There should be a check that no members are ever added this this group. GrouperAll should be able to READ this group.

1. If it is a union composite, and a factor is deleted, then replace the unioned deleted factor with the emptyGroup

2. If it is an intersection composite: then replace the intersected deleted factor with the emptyGroup

3. Complement. If it is the left factor, then just replace that factor with the emptyGroup. If it is the right factor then we have two choices:

a. replace both factors with the empty list
or b. just replace the right factor with the empty list

The right factor is the excludes list for a group, if that excludes list is deleted (perhaps unbeknownst to the app using the composite), I think the more practical thing to do (80/20 rule) is to just do "b". However, that might unwittingly give more access to the resource than was intended, so the most secure thing to do is to flag this as an error condition by doing "a".