add list_available_dates and get_aop_tile_extent functions + improved validation and update tests

Author: bhass-neon

src/neonutilities/__init__.py

@@ -1,5 +1,6 @@
 from .citation import get_citation
-from .aop_download import by_file_aop, by_tile_aop
+#from .aop_download import by_file_aop, by_tile_aop
+from .aop_download import by_file_aop, by_tile_aop, list_available_dates, get_aop_tile_extents
 from .tabular_download import zips_by_product
 from .get_issue_log import get_issue_log
 from .read_table_neon import read_table_neon