ENH: Remove uses of METAIO_EXPORT

If for some reason EXPORT macros are needed in future, we should have
TUBE_EXPORT so that exports are consistent throughout the library.

Change-Id: I529754a6db9da522d738d68c7b0e363446f74f0e
Merge topic 'RemoveMetaIOExport'

21bfa93 ENH: Remove uses of METAIO_EXPORT