v2.20.0

Fixed:

• Use chmod, not fchmod to support Windows, #636 ht @b2m
• Do not capture processor output in run_cli, #592, #638

Changed:

• Record version information in pg:MetadataItem, #637
• New method Processor.zip_input_files to uniformly handle files from multiple input file groups, #635
• Auto-invalidate derived pg:AlternativeImage images when changing coords, #639