Do not shard arrays that fit in one chunk #357

Reported in #349 (Z3). Related to #282.

With --enable-sharding, every array gets one shard that covers the whole array (conversion/utils.py:673-697). For 75 arrays in one product, the shard holds only one inner chunk (for example 9 × 9 meteorology arrays and 23 × 23 angle arrays). A reader then makes two requests (index, then chunk) where one is enough. In 66 arrays the shard is larger than the array (for example 2048 × 2048 for 1830 × 1830). That is valid, but it is not necessary.

Proposal

In build_dataarray_encoding, set shards = None when the array fits in one inner chunk.

Tests

Add a unit test for the shard rule. Regenerate the snapshots.