(Solved) bug with io and lazy loading #117

Noticed when working with the io of transformations and fixed in #100 (gonna close the issue when the pr is merged), but worth mentioning the bug here so that you are aware of that behavior.

The bug fix consists in the following: when an object is backed (=images, labels and points; not shapes or polygons, nor table (since it's an anndata)), after it is written, we immediately read that element from disk. If we don't do this there could be confusion cases like the following:

1. we write a SpatialData object with images/labels/points to disk
2. we load the object into a separate object, let's call it s. Now self.path
3. we write s to a different zarr storage; this will change self.path to the new file storage, but the in-memory object would still pointing to the old one.

I will now make a commit with tests against the behavior described above.