Work after switching to attrs about czml3

[noc0lour]

No, somehow I expected in poliastro to be able to stick results from the CZML_extractor directly into a CZMLWidget, which is a poliastro issue not a czml3 issue.

[noc0lour]

Note: The example in README is not working since CZMLWidget now expectes a Document() to be passed instead of a list. Now either a Document(simple) has to be passed or CZMLWidget has to do it in the initalizer.

Edit: For simple it works because it's a Document now, but it doesn't work for a list of packets. I don't know what would be the right way here. Maybe just a documentation issue.

[astrojuanlu]

@noc0lour I'd say that CZMLWidget should always expect a Document, do you see any inconsistency in the README or any docstring?

[astrojuanlu]

Oh, I see! Yep, CZMLExtractor should produce a Document with a proper Preamble.