Fixed zarr metadata read, minimal xml, and backend autoselect

[Nicholas-Schaub]

This PR fixes 3 bugs:

1. The ZarrReader backend did not properly read metadata due to an improper search for metadata.
2. Initializing any BioWriter always threw an error if no metadata object was supplied. It now properly initializes properly and generates minimal OME xml metadata.
3. The automated backend selector and backend validator would throw an error if any periods were in the filename. It now properly checks file extensions, even when a period might be present in the file name.