Fix for issue 733
Created originally on Bitbucket by gpellizz (Giuseppe Pellizzari)
Changesets for this Pull Request have not been imported, because it had been already declined on Bitbucket. Marked as closed by the import user.
Add a dict for name lookups to NamedStyleList (fix issue #733 (closed))
There are Excel files with many named styles. The management of style names was completely linear and needed quite some time. Adding a dicitionary uses a bit of memory but increases speed dramatically for these files.