Consider better ways to configure sharding across resolutions

[melissalinkert]

Setting a single shard size for all resolutions doesn't really work in general, unless the shard size is exceptionally small - at some point, the smaller resolutions will be smaller than the specified shard size. This leads to various errors and other problems as noted in point 1 of #290 (review) and #290 (comment).

At the same time, setting one shard size per resolution is likely too complex. We may wish to discuss with other parts of the NGFF community to determine a reasonable compromise.

[melissalinkert]

One idea from discussion with @normanrz and @sbesson is to have a single option that is the target shard file size (not XY/XYZ size), e.g. 1MB or 1GB.