parts of child needs are even occuring in the parent need

[PhilipPartsch]

If you build for example: tests\doc_test\doc_needflow_incl_child_needs the needs.json has the following content:

"STORY_1": {
    ...
    "parts": {
        ...
        "5EA": {
            "content": " subsubstory 3",
            "document": "index",
            "id": "5EA",
            "is_need": false,
            "is_part": true,
            "links": [],
            "links_back": []
        }
    },
    ...
    }

but subsubstory 3 is been defined in STORY_2 and shall not occure in STORY_1. See https://github.com/useblocks/sphinx-needs/blame/master/tests/doc_test/doc_needflow_incl_child_needs/index.rst#L16

[PhilipPartsch]

Issue is here the function checks if a need_part is in the content, and so even in content of Childs.