Add XY metadata

[joaomamede]

Added code to include X and Y stage position in the metadata dictionary.

[rbnvrw]

@joaomamede there are some failing tests (see for example https://github.com/rbnvrw/nd2reader/runs/4170041379?check_suite_focus=true) ValueError: bytes length not a multiple of item size I think it is unrelated to your changes, but I'd like to fix it first before merging just to be sure. I hope to be able to look at it next week, or if you have an idea what it could be, feel free to look at it in the meantime.

[joaomamede]

I had done the tests locally, but I couldn't find any of the errors to be connected with the changes. I just changed the metadata attribution in the dictionary, not even the file reading.

I has been working well in linux and windows by the way.