Parse zarr v2

[TomNicholas]

@norlandrhagen this is in theory all that would be needed. But when I actually run it I hit some bugs. e.g.

TypeError: Cannot convert object None with type <class 'NoneType'> to a scalar compatible with the data type Int16(endianness='little').

• Closes Virtualize Native Zarr V2 format #565
• Tests added
• Tests passing
• Full type hint coverage
• Changes are documented in docs/releases.rst
• New functions/methods are listed in api.rst
• New functionality has documentation

[TomNicholas]

fyi @d-v-b I think this is the piece of API from zarr-developers/zarr-python#3257 that we would ideally like to be exposed as public and stable in zarr-python.

[d-v-b]

that function will probably need some time to stabilize but in general 💯 to making it public

[d-v-b]

TypeError: Cannot convert object None with type <class 'NoneType'> to a scalar compatible with the data type Int16(endianness='little').

That's a symptom of converting a V2 array which had fill_value: null to v3, where the fill value can't be null. You can handle this case by using the default fill value, which is associated with the data type via default_scalar method