This reverts commit 4bf90069.

This changeset introduce support for Gdal derived subdatasets.

Changes include:
- A new private const method to get the real source name from the derived dataset filename
- An update of the TestFileExistency() method to check for existency of the real source name instead of derived dataset filename
- A patch to make sure OSSIM tries to open the real source name instead of the derived dataset.

If derived datasets are not supported by gdal, ImageFileReader will end with GDALImageIO CanRead saying false, so there should be no impact.

ENH: Ensure RAM parameter is used by persistent filters (should be applied likewise for all applications)

BUG: Actual number of threads may be less that GetNumberOfThreads() if requested region has nb lines < GetNumberOfThreads()

ENH: Modify prototype for AddImage method so as to accept any ImageBase pointer. Should not break the API.