Better way to handle Tiff/Exif Sub IFD

[dragon66]

For now, there is a one on one approach when it comes to manage Tiff/Exif fields which are pointers to sub IFDs. For now the approach is one on one which is not flexible and awkward when adding to the library.

A better way needs to be find for this.

Some thoughts:

1. Add new boolean ifSubIFD to Tag class.
2. Override in each tag class to provide default implementation - new Java version makes this easier to add default to Tag interface.
3. Override in each tag definition which is points to a sub IFD.
4. Use this information in reading and processing IFD.