[hiopy] refactor variable handling and distribution

This MR refactors how variables are handled in the worker.

• instead of monkey patching the zarr.Array the group of a variable is generated from the store and the name of the variable
• hiopy components are not split up by variables any more. If not enough chunks are available a warning is emitted and the group is distributed on fewer processes
• variables can be split up into different components manually by setting the hiopy::var_group attribute
• for each spatial chunk size a separate coyote instance is used to ensure that the domain decomposition is correct
• minor improvements

hiopy::parent must now be given as the "absolute path" of the group within the same dataset (store).