Fix common folder not included in built wheel (#30)

* Add __init__ file to common folder

* Add unittest for folder structure

* Resolve path sooner

Author: misimpso

src/wktplot/__init__.py

@@ -1,3 +1,3 @@
 from .plots.standard import WKTPlot  # noqa: F401
 
-__version__ = "2.3.0"
+__version__ = "2.3.1"

src/wktplot/common/__init__.py

@@ -0,0 +1,15 @@
+from pathlib import Path
+import os
+
+
+SOURCE_PATH = (Path(__file__).parent / ".." / ".." / "src" / "wktplot").resolve()
+
+
+class TestFolderStructure:
+    def test_source_structure_has_init_files(self):
+        assert SOURCE_PATH.is_dir()
+        for directory, _, files in os.walk(SOURCE_PATH):
+            directory = Path(directory)
+            if directory.name == "__pycache__":
+                continue
+            assert "__init__.py" in files, f"Folder missing __init__.py file, {directory}"