Selecting specific stains when reading merscope files into a sdata object.

[omiethescientist]

The merscope image files that I use are currently quite big, and it is often useful for me to only select specific stains. Currently it is hard coded in the merscope function to read in all stains. However, it is an option in the _rioxarray_load_merscope function. It would be nice to set this ourselves as one of the kwargs or as a keyword argument in the merscope function itself.