loose / strict (core) / strict(extras)

[d-v-b]

there are 3 levels of validation sensible for array metadata:

• loose: syntax-only checks that ensure that the metadata document is well-formed
• strict(core): syntax checks + validation that the metadata complies with the data types + codecs + chunk grids + fill value etc in the core spec
• strict(extra): above checks but also include data types + chunk grids + codecs from zarr-extensions

I think one set of arrayspec / groupspec classes for each level would probably be useful for consumers who want to ensure that they are operating on a defined subset of zarr capabilities.