Backwards compatibility of session.volview.zip files

[PaulHax]

VolView should load session.volview.zip files with older manifest.json schema's, if possible.

#444 bumped the manifest.json version from 2 to 3. The JSON shape changes are minor https://github.com/Kitware/VolView/pull/444/files#diff-5bd5ad26fa2f3b6ccf22510d0eb4ac2339bb75238a01436a734cc474ee09a465

Approach?

Add another Pipeline that checks the manifest version and tweeks the the JSON for compatability with the next version. Does that here:
https://github.com/Kitware/VolView/blob/main/src/io/import/processors/restoreStateFile.ts#L237