Add API call to return metadata struct

[selamberson]

(This issue was migrated from Kestrel's issue tracker. It was originally posted by Eric Lynch).

It would be really useful if avmeshlib gained the ability to return a persistent object containing all the mesh metadata.  I frequently write scripts to fix problems in meshes (working around bugs in CreateVolumeMesher).  Usually, I don't want to change mesh metadata at all.  However, if I want to write a new mesh file with the same metadata as an old one, I have to make dozens of calls to avmeshlib to retrieve the metadata, one item at a time, then dozens of calls to set it in the new mesh.  It would be awesome if I could do something like:

rv, avid = avmesh.read_headers('in.avm')
rv, meta = avmesh.get_all_metadata(avid)
# At this point, I can look at meta.nNodes or meta.coordinateSystem
# Read heavy data
avmesh.close(avid)
...
# Manipulate connectivity or whatever
...
rv, avid = avmesh.avm_new_file('out.avm')
rv = avmesh.set_all_metadata(avid, meta)
# Write heavy data...
avmesh.close(avid)