Chunks skipped in l3.build_dataset if coordinate non-contiguous

[j-haacker]

Describe the bug
For chunks at the margin of glacierized areas, some time steps are missing because they
don't hold data. In that case, an error occurs and a fallback routine dumps the data to a
temporary file.

To Reproduce
Steps to reproduce the behavior:

1. l3.build_dataset on virtually any region

Errormsg

The auto-detected region of coordinate 'time' for writing new data to the original store
had non-contiguous indices. Writing to a zarr region slice requires that the new data
constitute a contiguous subset of the original store.