gdal_misc_test.cc: Fixes for GdalDataTypeUnion, add GDALDataTypeUnionWithValue, add GDALFindDataType
gdal_misc_test.cc: Fixes for GdalDataTypeUnion, add GDALDataTypeUnionWithValue, add GDALFindDataType

TODO's mark trouble spots.  There are probably more waiting to be found.

schwehr/gdal-autotest2,schwehr/gdal-autotest2