Added reading of IFD tag type

[radarhere]

Helps #4940

Pages 15-16 of the TIFF specification list tag types from 1-12. It also states that

Warning: It is possible that other TIFF field types will be added in the future. Readers should skip over fields containing an unexpected field type.

However, the image from the issue has a tag of type 13. Why? It is instead following TIFF Specification Supplement 1 which lists 13 as the type for a 'New Tag' on page 4. It describes it as

TIFF data type 13, “IFD,” is otherwise identical to LONG, but is only used to
point to other valid IFDs.

At the moment, as it is an unknown tag type, it is simply skipped by TiffImagePlugin.

So this PR adds IFD as a tag type, read in the same way as a LONG.