Remove io/exporter capabilities from Origins

This is out of the scope of Origins. In addition it's immediate capabilities
are supported by graphlib.

Updated graphlib to 0.9.4 which supports bi-directional traversal of
relationships.
Add back `element_preceding_header` in SELECT for REDCap MySQL backend

This was removed in b435bb82 to prevent redundancy in the field attributes,
however the value is still needed for filtering. The attribute is now removed
from the attributes before returning.