refactor code into modules

src/pg_rad/exceptions/exceptions.py

@@ -0,0 +1,8 @@
+class ConvergenceError(Exception):
+    """Raised when an algorithm fails to converge."""
+
+class DataLoadError(Exception):
+    """Base class for data loading errors."""
+
+class InvalidCSVError(DataLoadError):
+    """Raised when a file is not a valid CSV."""