fix performance of geometry test

This [ticket](http://trac.mantidproject.org/mantid/ticket/10487) investigated a performance regression introduced in [performance build 527](https://github.com/mantidproject/mantid/commit/f94f814ce5e1d89785aece20fa280c2963b94b40). I changed back from boost::unordered_multimap to std::multimap and performance appears to have improved slightly. 

For testing, I think a code review is sufficient.