[Bug]: iframe content not accounted for when calculating ecoindex

### What happened?

ecoindex_cli does not take the content of iframes into account when calculating the ecoindex of a page.

I'm using the following files as a [testcase.zip](https://github.com/cnumr/ecoindex_cli/files/10842098/testcase.zip), served locally on port 8080.
I analyze the page using the following command and get this result :
```
ecoindex-cli analyze --url http://127.0.0.1:8080/testEcoIndexIFrame.html --export-format json --outputfile ./result.json
```

result:
```
[
{
"width": 1920,
"height": 1080,
"url": "http://127.0.0.1:8080/testEcoIndexIFrame.html",
"size": 2.511,
"nodes": 6,
"requests": 3,
"grade": "A",
"score": 97.0,
"ges": 1.06,
"water": 1.59,
"ecoindex_version": "5.4.1",
"date": "2023-02-27 17:50:40.593434",
"page_type": null
}
]
```

The number of DOM nodes detected by ecoindex-cli is 6, which maps the content of the main html file, but does not include the content of the html inside the iframe.
The number of requests and the size seems to be correct though : when I increase the subPage.hml page's size, the size reported by ecoindex-cli increases as well.

On some real life pages, this can make a huge different in the final rank obtained when analyzing a page (from G to E rank).

### Version

Above 3.6

### What OS do you use?

Linux

### urls

_No response_

### Relevant log output

_No response_

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct