Allow manually overriding which series to use in TiffSlide

[ap--]

This came up as a request (initial solution) from #21
While this might usually not required if the default behavior of TiffSlide returns the same series as openslide, it could be an interesting feature in case a user wants to change the default.

I'll have to think about it a bit more, if it's worth supporting this for any format.

[One-sixth]

I give an example. Sometimes the default behavior selects the incorrect series, and manual selection allows us to quickly fix it without having to modify the library code. Because modifying the library code is difficult for some people.

[ap--]

I completely agree.
I wonder if specifically selecting a series should make TiffSlide fall back to the generic tiff-route though.

Do you have an example of the <scn> xml for a file in which the auto selection fails?

[One-sixth]

I don't have any more examples, because I just learned about and started experimenting with tiffslide last week.

[ap--]

I just created a new release v1.1.0 with your added support for scn.
Please try that version and if there are cases in which the correct series is not found, we'll work out how to best implement this functionality.

Cheers,
Andreas 😃

[One-sixth]

Cheers!