Fixed:
- OcrdExif: PNG metadata extraction was broken, #395, #396
- Remove the trivial and error-prone image caching feature in resolver, #399
- When creating files with workspace.add_file, single-component file paths (i.e. just the basename of a file) were treated as directories, #404
- When downloading files to a workspace, check first whether those files to be added already exist on disk and are within the workspace directory., #404