Some CTopo* APIs are slow

[rbx]

When using this code to generate a topology file:
https://github.com/FairRootGroup/ODC/blob/0.66/examples/odc-epn-topo.cpp
With this script (cmd.sh) with sub-topologies as arguments (all the other files):
https://gist.github.com/rbx/c1eed6a115a14fef33ace471f2de599a

The runtime on my machine is around 6 seconds. Which seems way too high for merging a few XML files.

It looks like the very long task <exe> line takes a long time to parse. If I reduce it to just the executable name, then the generation is much quicker - only 157ms!

With the above code you should be able to reproduce this, it depends on nothing except boost and DDS.