Global spatialdata format version number?

[LucaMarconato]

@nilseling is working on extending existing R visualizers to support NGFF and the SpatialData format. He suggested to just add a version number for each element type, but to also bump a general version number when any of the element type changes in version.

CC @giovp

[LucaMarconato]

With #711 a different approach is used (so closing this issue). The SpatialData object is versioned at the root level, but the format is independent of the various elements. In this way the elements can be parsed independently, and if something changes at the root level this is reflected in the version number.

Practically, now we have a new 0.1 format for the root level. Nothing changes except for the new ability for the user for setting custom json-like data in SpatialData.attrs, which is automatically serialized in .zattrs.