Feature: option to deal with missing values

[huard]

At the moment, header_extract only returns data for attributes that it finds. If an attribute is missing, it's just not part of the data["body"]["properties"] dictionary. This seems to be the common behavior among input plugins.

If there are variations in attributes from one file to the next, the data payload will vary. This would be problematic for example with a CSV output plugin, where we'd expect the same columns on every row.

One option is to modify all input plugins to return None for missing attributes. Another would be to have a more elaborate output mechanism, with three steps:

• backend.open : initialize backend
• backend.export : export data payload
• backend.close : finalize backend (write to disk)

The close method could be used to aggregate all properties and fill missing values.