don't crash when encountering Things

Less crash more skip (#14)

* Skip more crash less

Skip, rather than crash when encountering Thing.

Also:
* More supported person roles (can now explicitly skip some)
* More supported places roles (can now explicitly skip some)
* Don't assume alternative id exists
* Don't assume timespan exists for an event.

* Add support for tags

Tags are harvested separately but combined with subjects for mapping
purposes.

For commons purposes they are combined in keyword generation but
separated under "original description".

Also fixing various assumptions of data existance during Mapping.

* Warning instead of a new unused parameter

Also fix typo