Updated Tags
- Tags now use enums instead of simple strings (`pytiff.tags`).
- Fields with a scalar do now return a scalar instead of a numpy array of length 1.
Page Handling
- you can no longer iterate over a Tiff object.
- instead a Tiff object has a `tif.pages` attribute, that returns a list with each page.
- the manual way via calling `tif.set_page(1)` is still valid.