Assimp metadata

Based on https://sourceforge.net/tracker/?func=detail&aid=3539265&group_id=226462&atid=1067634

Adds aiMetaData API to API. Each node now keeps an mMetaData member, which offers a key-value store for arbitrary meta data in string format.

Adds metadata support to the Ifc loader. Ifc property sets are directly transferred to node meta data.