Miscellaneous corrections to Pack

[freakboy3742]

This PR addresses some issues with the Pack layout algorithm that were found in the process of developing #1956.

• Converts the Pack tests to pytest
• Adds tests to get 100% branch coverage for Pack, including CSS rendering
• Allows a node to have an explicit width/height of 0.
• Alters the default width/height of a node to be NONE, rather than 0. This matches CSS, and is needed to ensure that nodes have a "floating" width/height unless a size is explicitly provided.
• Ensures that if a node has an intrinsic size of 0, it is honoured, rather than being ignored.
• Replaces the special handling of root as an "expanding" node with a more generic handling that ensures that column boxes won't have a their width collapsed, and row boxes won't have their height collapsed. This has a signifiant impact on alignment tests (in particular, the row/column alignment and rtl tests); a row box in a row box will now preserve height on the inner row box, and as a result, centre alignment on the inner row box will introduce a larger vertical offset - children of the inner row box will be vertical centered on the window, not just the content of the inner row box. A similar case exists for content in column boxes.

The test cases for the tutorials + beeliza are unchanged.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

I still can't claim to have a strong understanding of the layout algorithm, but everything looks reasonable.