they will call get_$group($field) on get and set_$group($field, $value)
on set.
+ If you want to mimic Class::Accessor's mk_accessors $group has to be
+ 'simple' to tell Class::Accessor::Grouped to use its own get_simple and
+ set_simple methods.
+
@fieldspec is a list of field/accessor names; if a fieldspec is a scalar
this is used as both field and accessor name, if a listref it is
expected to be of the form [ $accessor, $field ].