Filesystem harvester

We may want to harvest a set of existing ISO19139 metadata we already have, but that are not served by a CSW server. In this case we'd put the metadata files inside a directory in the local filesystem, and harvest them from there.
This harvester would also be useful in case the remote server can only serve metadata in push mode: the server running CKAN would offer an exchange directory, where the remote server would put its files, and where this harvester would harvest from.

No files, tests, or entry points are named in the issue. First clarify the filesystem layout, ISO19139 discovery and validation requirements, scheduling or trigger behavior, and how harvested records should be reported; the work is done when those requirements and acceptance checks are defined.

Written by the indexing model from the issue text.