-Ystatistics accepts a list of phases for which to print stats

Also cleaned up handling of defaults in MultiChoiceSetting.
It's evolutionary, but unwinds a couple of historical artifacts, from when I extended MultiString.  First of all, there is no notion of "choosing", as if MultiChoice could have no choices!  That was a leftover.  So, it doesn't extend MultiString. Then it's obvious that it should take an enum and value is a ValueSet and not a List[String].