Use functions instead of methods for the interface functions (#334)

* starting refactor

* remove stuff

* unindent

* remove self from function signatures

* remove self.

* remove staticmethod

* set up symlinks

* some linting

* test for bad H3Shape

* symlink note

* just put everything in __init__.py

* reorganize conversion functions

* rename _api_funcs.py to _convert.py

* use lazy import of numpy so we don't have to special case h3.api.numpy_int

* DRY

.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