Add a helper method to create an aggregate function for string

[spelletier]

in ERXEOControlUtilities.

[paulhoadley]

If I'm reading this correctly, it's a cover method for _aggregateFunctionWithQualifier() where you supply String.class to that method, and then cast the result to a String?

[hprange]

It seems so. What bothers me about this pull request is the Javadoc.

select MAX(AGE) from User where name like 'M*'

This query doesn't seem like a good example of when to use this method. @spelletier what's your use case? Can we improve the Javadoc to reflect it?

[spelletier]

@paulhoadley, this is a simple wrapper indeed. @hprange, I adjusted the javadoc with a real usage.