allow zarr writer to do incremental writes of single timepoints

[toloudis]

Link to Relevant Issue

Chantelle had a use case where she wanted to write out a time series zarr but could only prepare the source array sequentially, one timestep at a time. She wanted to use the writer to write sub-arrays incrementally.

Description of Changes

So I am adding a toffset parameter to the zarr writer. The writer will then attempt to write the given time series starting at the given t offset.

In this way you can call write_t_batches_from_array more than once, in a loop, passing in smaller amounts of t at a time.

A unit test is added to show how this should be used. You can just call write_t_batches_array in a loop now.

A future generalization of this could be to pass in even smaller parts of the array and give the writer a tczyx offset of where to start writing the input data.

[evamaxfield]

Nice!! Only comment would be that it might be nice to have an example of this in the top level writer docstring