Add an append method that takes frame data, size, and shape

[aliddell]

I updated Zarr::append() to call the new Zarr::append_frame(). Multiscale logic goes from append() into append_frame(), so I needed to retool Zarr::write_multiscale_frames_() to accept uint8_t* and size_t instead of VideoFrame.

I also updated Writer::write() to both take uint8_t* and size_t instead of VideoFrame, and also return a size_t of the number of bytes written in anticipation of establishing a contract around writing (separate PR). Because of this change, I had to update the unit tests around {Writer,ZarrV2Writer,ZarrV3Writer}::write(), and I took the opportunity to define constant values instead of hardcoding numbers. The tests are otherwise the same.

Also the unit tests are slightly reordered for aesthetic reasons.

[jeskesen]

looks good to me.