Optimize IfcFile::entitiesByReference() by reserving capacity for IfcEntityList in advance.

Around 18.4 % speed-up (avg. of first 2000 calls) when converting a somewhat large (176 MB) file to XML.

src/ifcparse/IfcUtil.cpp

@@ -48,6 +48,7 @@ void IfcEntityList::push(const IfcEntityList::ptr& l) {
 	}
 }
 unsigned int IfcEntityList::size() const { return (unsigned int) ls.size(); }
+void IfcEntityList::reserve(unsigned capacity) { ls.reserve((size_t)capacity); }
 IfcEntityList::it IfcEntityList::begin() { return ls.begin(); }
 IfcEntityList::it IfcEntityList::end() { return ls.end(); }
 IfcUtil::IfcBaseClass* IfcEntityList::operator[] (int i) {