ENH: Define specific dimension sizes as enums instead of variables

Avoids warnings for variables only used as template arguments.

Change-Id: I2338f070a38ad81699e50903c80014c554240d94
Merge topic 'EnumDimension'

b5883e2 ENH: Define specific dimension sizes as enums instead of variables
ENH: Remove use of ITK_EXPORT

The definition of ITK_EXPORT is always empty so it is redundant.

Change-Id: Ic58b70b857fd1361cee0ae1156b0e068578a76ca
Merge topic 'RemoveITKExport'

1ca2d1b ENH: Remove use of ITK_EXPORT