Make DataFrame & Pipelines truly immutable

[norberttech]

Currently run method triggers DataFrame clone internally, the idea behind that was to always duplicate the dataframe in order to make it reusable. However PHP clone is shallow, I tried to use deep copy library however readonly properties are not allowing for it.
The solution for this problem is to make DataFrame and Pipelines truly immutable, meaning that adding any new element should create entirely new DataFrame.
One thing to remember is that extractors should be always recreated as well, otherwise all instances of DataFrame will keep reference to the same extractor and only one instance will be able to use it.

This test should be a confirmation that dataframe is truly immutable:

<?PHP

        $df = (new Flow())
            ->read(
                From::array([
                    ['date' => '2023-01-01', 'user' => 'user_01', 'commits' => 1, 'active' => false],
                    ['date' => '2023-01-01', 'user' => 'user_02', 'commits' => 2, 'active' => true],
                    ['date' => '2023-01-01', 'user' => 'user_03', 'commits' => 3, 'active' => true],
                    ['date' => '2023-01-02', 'user' => 'user_01', 'commits' => 4, 'active' => true],
                    ['date' => '2023-01-02', 'user' => 'user_02', 'commits' => 5, 'active' => true],
                    ['date' => '2023-01-02', 'user' => 'user_03', 'commits' => 6, 'active' => true],
                ])
            )
            ->filter(ref('active')->isTrue());


        $this->assertSame(
            [
                ['date' => '2023-01-01', 'commits_sum' => 5],
                ['date' => '2023-01-02', 'commits_sum' => 15],
            ],
            $df->groupBy(ref("date"))
                ->aggregate(sum(ref('commits')))
                ->fetch()
                ->toArray()
        );

        $this->assertSame(
            [
                ['date' => '2023-01-01'],
                ['date' => '2023-01-02'],
            ],
            $df->fetch()->toArray()
        );