Prevent overwriting existing files

[normanrz]

I tried to use ngff-zarr to convert a 0.4 image to 0.5. Accidentally, I used the same path for input and output. ngff-zarr did not fail and overwrote the original image. I am not sure if this is intended. I would have expected to get an error when trying overwrite existing files (without an --overwrite flag or similar).

[thewtex]

Yes, this is intended, providing a default behavior that most folks want. We could print a warning or a flag that will error on overwrite.

[thewtex]

@normanrz a check that the input and output dirs are not the same is added in #223