Use functions instead of methods for the interface functions

.coveragerc

@@ -0,0 +1,5 @@
+[run]
+omit =
+    */h3/api/basic_int/__init__.py
+    */h3/api/memview_int/__init__.py
+    */h3/api/numpy_int/__init__.py

src/h3/api/__init__.py

@@ -3,3 +3,4 @@
 from . import basic_int
 from . import basic_str
 from . import memview_int
+from . import numpy_int