Add a way to create schemas

Hi,

The library allows to create paths, but apparently doesn't provide any way to add schemas.

Doing something like:
`$objOpenapi->components->schemas['foo'] = new Schema([
'type' => Type::INTEGER,
]);`

gives a warning and the element is not created:
PHP Notice: Indirect modification of overloaded property cebe\openapi\spec\Components::$schemas has no effect in G:\test.php on line 119

Am I missing something ?

If not, it would be nice if we could create dynamically elements using this library.

Thank in advance