ENH: update the call to RWV examine() - now takes one file, not list
ENH: add RWV as derived item

It should be added only once at the time it is created, and it will be created
when there is a PET series that does not have a matching RWV series.
BUG: fix typo
BUG: add derived RWV to the appropriate loadable
Merge pull request #1 from fedorov/cleanup

Minor modifications