Resolves #520

In addition to the methods described in that issue, I have also added get_ifd(), for reading specified subdirectories -

• The Interop EXIF subdirectory, to help Can't get "icc_profile" #3270. Reference documentation -https://www.awaresystems.be/imaging/tiff/tifftags/interoperabilityifd.html
• The Makernote subdirectory, to resolve No way to read Parallax tag from stereo MPO files #2081

Regarding #2081 -

MPO files can have EXIF data for separate images (Page 9 of http://www.cipa.jp/std/documents/e/DC-007_E.pdf). This PR changes im.info["exif"] to be specific to the current MPO frame.

Within the EXIF data of the second image in files in #2081, there is a Makernote EXIF tag containing parallax data. However, the MakerNote tag does not have consistent structure across MPO images - it is manufacturer specific. See https://www.awaresystems.be/imaging/tiff/tifftags/privateifd/exif/makernote.html and http://www.exiv2.org/makernote.html.

The issue provides Fujifilm MPO images. The specification of the Fujifilm MakerNote tag can be found at http://www.ozhiker.com/electronics/pjmt/jpeg_info/fujifilm_mn.html#Fujifilm_Tags, and the parallax tag value can be found at https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/FujiFilm.html. I have added one of the images from the issue.

Within the test suite, there are Nintendo MPO images. See https://3dbrew.org/wiki/MPO and http://owl.phy.queensu.ca/~phil/exiftool/TagNames/Nintendo.html for information about their MakerNote tag.

The test added in this PR reads parallax data from both types of files.