Define reader usable by Xarray

[maxrjones]

cc @hrodmn

[hrodmn]

Thanks for sharing this! After our discussion this morning I experimented with a reader that caches the file to a MemoryStore then handles the read/seek operations on the file in memory:

class ObstoreMemCacheReader(ObstoreReader):
    _reader: ReadableFile
    _memstore: MemoryStore

    def __init__(self, store: ObjectStore, path: str) -> None:
        """
        Create an obstore file reader that caches the specified path
        in a MemoryStore then performs reads from the file in memory.
        ----------
        store
            [ObjectStore][obstore.store.ObjectStore] for reading the file.
        path
            The path to the file within the store. This should not include the prefix.
        """
        self._memstore = MemoryStore()
        buffer = store.get(path).bytes()
        self._memstore.put(path, buffer)
        
        self._reader = obs.open_reader(self._memstore, path)