(Running the dev branch)

When reading, modifying, then writing a design, I found that the "grid" key in the scadnano file becomes mangled.

It appears that the problem is:

1. The grid value in the scadnano file is a string, without any "Grid." prefix.
2. When read with Design.from_scadnano_file, the grid value remains a string, rather than being interpreted as a Grid Enum.
3. When written, str(grid) is written with the first 5 characters removed, to remove "Grid.". But as a string, instead of a Grid, it doesn't have the "Grid." prefix, resulting in either a blank string (with "none") or "e" (with "square"), which then crashes scadnano itself.

I'm a bit confused as to why it appears this is apparently not being caught by type checking, though it might have to do with the underlying Enum representation being a string.

I quickly wrote a modification to the writer, in the fix-grid branch on my fork, but I don't think this is actually the correct way to fix the problem.