[ENH] Metadata for data files

[janezd]

@astaric, you meant something like this?

Orange already read and wrote .metadata files, but in pickle format. This is highly inconvenient since the user can't edit it. I changed the format to a (very) simplified YAML: the file is expected to contain a single block mapping. Data is read as string. If we wish, we can either try to evaluate it (with fallback to string) or use PyYAML, although I don't think we need it.

[astaric]

@BlazZupan, @markotoplak does this change break your use of metadata?

[kernc]

It does break reading any existing metadata files. Might be avoided excepting to old unpickling.

[codecov-io]

Current coverage is 88.67% (diff: 100%)

Merging #1603 into master will increase coverage by <.01%

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[janezd]

I had it implemented (the other way around, I first checked whether it's a pickle). Then I "discovered" (don't ask me how) that nobody uses .metadata and decided to switch to a "better" format while it's still time.

I re-added it. For writing, pickle is used if there already exists a metadata file that is a pickle, or if the current attributes are not an OrderedDict with strings as keys and values.

This is imperfect, but I think that having a plain text format is better than a binary - when possible. I wouldn't like to have two metadata files. Any better ideas?

[janezd]

Fixed as discussed:

• when loading, it tries to unpickle, then to read as "YAML"
• when saving, it uses "YAML" if the attributes are a dict with string keys and values