fix: `PodioOutput` metadata branch `CollectionTypeInfo` is not completely filled

Close #104 (closed)

PodioOutput::createBranches is called twice, but the CollectionTypeInfo branch is created within (twice). This MR:

• changes local collectionInfo to private member m_collectionInfo, so its state is preserved between createBranches calls
• creates the CollectionTypeInfo branch after the createBranches calls

Credit also to @chchatte