Add default values support to DwcaWriter

Ready for review.
The implementation allows to set default values before, during or after calling `addCoreColumn(...)` and `addExtensionRecord(...)`.

The cost of that is a check on all `addCoreColumn(...)` and `addExtensionRecord(...)` calls to ensure we do not overwrite a default value. An alternative might be to throw an `IllegalStateException` on the `close()` if we have a value **and** a default value for the same rowType/term.

What do you think?