feat: Add data processing module

Signed-off-by: Mohammad Tayyab <[email protected]>

Author: Mohammad-Tayyab-Frequenz

RELEASE_NOTES.md

@@ -16,3 +16,6 @@
 
 * Replaces multiple duplicated plot functions with a single reusable one.
 * Handle empty weather/reporting dataframes gracefully to avoid transformation errors. The "Solar Maintenance" notebook is updated accordingly.
+<!-- Here goes notable bug fixes that are worth a special mention or explanation -->
+
+* Add reusable, modular data processing functions