cellvoyager.StackedTile: consider making _paths field public

[imagejan]

For our use of faim_hcs in faim-wako-searchfirst (to assemble tiled first-pass acquisitions), we need to get the path of the first tile (field F0001) to derive the name of the .csv file.

It would be good to have access to paths as a public field, in analogy to Tile.path, so we don't have to rely on accessing a private _paths field.

(This is of course not blocking me at all, I just file this issue to consider for API stability...)

[tibuch]

I am not very happy with how it looks at the moment: path vs _paths
Refactoring this part into something consistent would be awesome!

[imagejan]

Closing in favor of #118.