Unique rel_type per metaedge
Unique rel_type per metaedge

Append abbreviation to rel_type so every metaedge has a unique rel_type.
Previous implementation only appended the abbreviation for metaedge to metarel
conversion.

Add caching for metanode to label conversion, metaedge to rel_type conversion,
and metaedge to metarel conversion.

dhimmel/hetio