$group is the name of the accessor group for the generated accessors; they
will call get_$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 ].