Open files containing multiple datasets

[JcGKitten]

Hey,

The current release doesn't allow users to choose which dataset they want to load if a file contains multiple datasets or am I missing something?

It seems like all the functions are already ready to read a selected dataset, but there is no check for the nextdata keyword to recognize if there is more then one dataset available. I could try to open a pull request for this, but wanted to check in with you first.

Best wishes Max