feat: add app file for CLI

Author: cadenmyers13

src/diffpy/cmi/app.py

@@ -0,0 +1,53 @@
+import getopt
+import sys
+
+
+def usage():
+    """Print short help message."""
+    print(
+        """\
+
+DiffPy-CMI is our complex modeling framework. It is a highly flexible library
+of Python modules for robust modeling of nanostructures in crystals,
+nanomaterials, and amorphous materials.
+
+Docs: https://www.diffpy.org/diffpy.cmi
+
+Usage:
+    diffpy-cmi [--version] [--help]
+
+Options:
+    -V, --version     Show version and exit
+    -h, --help        Show this message and exit
+"""
+    )
+
+
+def version():
+    from diffpy.cmi.version import __version__
+
+    print(f"diffpy-cmi {__version__}")
+
+
+def main():
+    try:
+        opts, args = getopt.gnu_getopt(sys.argv[1:], "hV", ["help", "version"])
+    except getopt.GetoptError as err:
+        print(f"Error: {err}", file=sys.stderr)
+        usage()
+        sys.exit(1)
+
+    for opt, _ in opts:
+        if opt in ("-h", "--help"):
+            usage()
+            return
+        elif opt in ("-V", "--version"):
+            version()
+            return
+
+    # Default behavior (if no arguments)
+    usage()
+
+
+if __name__ == "__main__":
+    main()