Implement attribute to reorganize data

[KES777]

->selectall_hashref( $q, 'property_id', { Merge=>{} }, @values )
If here two rows with same property_id I would expect:

{
  6699 => {
    inv_num     => [ 'DDFD', 'ASDF' ],
    field2      => [ 7, 333 ],
    property_id => 6699,
  },
}

Or, depending, on requirements: {Merge => 'inv_num'}

{
  6699 => [ 'DDFD', 'ASDF' ],
  3433 => [ 'XYZ',  'ZZZZ' ],
}