Moved code from calculate notebook into 2 python files: MetricSet.py which contains the metricset class and metrics.py which contains the code to update our metrics tsv files. These are located in the src folder.

Removed calculate notebook and updated report notebook to call the new modules.  

Renamed report notebook to 'monthly_report' to be consistent  with 'quarterly_report'. The readme file was updated to reflect this new process.

BUG: T368218

See merge request !20