Introduce `Application` class and reorganize module structure

src/metar_navigate/__init__.py

@@ -0,0 +1,5 @@
+from metar_navigate.application import Application
+from metar_navigate.command import main
+
+
+__all__ = ["Application", "main"]

src/metar_navigate/application.py

@@ -0,0 +1,7 @@
+
+
+class Application:
+    def __init__(self):
+        pass
+    def run(self):
+        pass

src/metar_navigate/command.py

@@ -1,2 +1,5 @@
+from metar_navigate import Application
+
 def main():
+    app = Application()
     print("Hello World")