1. in structure node, delete layNum; 2.in structure IX_FileHdr, add treeLayerNums and change numMaxEntries to orderOfTree; change ix_manager.cc to adapt the change of structure; 3. change function collapseRoot in ix_indexhandle.cc; 4. modify and add comments to be more clear; 5 move the definition of structure nodeInfoInPath from ix_indexhandle.cc to ix_internal.h;  6. add a parameter pathDepth in some functions in order that we don't have to keep a record of layNum for each node 7. correct some indice problems in the for loop