Fix a bug in + simplify the FreeDataset method.
Fix a bug in + simplify the FreeDataset method.

Conceptually simplifies the search for datasets/renderers which
match the given parameters.  Bail out earlier when an error
occurs.  Most importantly, conditionally build the dataset string
name, taking account that the pVolumeDataset argument could be
NULL.

git-svn-id: 353be9ac11a1e711854a13efcc2fc17d9d6b83e7@919 c36c8488-0289-0348-9b64-b301f74bd9a7

BlueBrain/Tuvok,BlueBrain/Tuvok,BlueBrain/Tuvok