Feature/implement expressions

[romaninsh]

Implements expressions:

$m = new Model_Invoice($db);
$m->addFields(['total_net', 'total_vat']);

$m->addExpression('total_gross', '[total_net]+[total_vat]');
$m->load(1);

echo $m['total_gross'];

Documentation: https://github.com/atk4/data/blob/feature/implement-expressions/docs/expressions.rst

• make sure Implemented SQL conditions #35 is merged
• make sure Feature/implement relations #36 is merged

resolves #29, #30

[skondakov]

Seems good to me. Great work!