to_zarr() fix compression

Author: dmitriyrepin

_dev/DEVELOPERS_NOTES.md

@@ -4,7 +4,12 @@ src/mdio/schemas/v1/dataset_serializer.py
 
 ## Issues encountered
 
-1. Non-zero size of the serialized data files
-2. Not clear how to properly set `compressor`, `dimension_separator`, and `fill_value`
-    * Should `fill_value` be a part f the model?
-3. For image_inline chunks[2] are somehow different?
+1. FIXED: Non-zero size of the serialized data files
+2. FIXED: Not clear how to properly set `compressor`, `dimension_separator`, and `fill_value`
+3. FIXED: For image_inline chunks[2] are somehow different?
+
+4. `fill_value` for StructuredType is set to null, but "AAAAAAAAAAAAAAAA" is expected
+
+## TO DO:
+* Add more unit tests for internal functions
+* Add a trest comparing expected and actual .zmetadata for the serialized dataset