Library Statistics - 1. Turnover rate of the Library collection

[JakobMiesner]

Library Statistics

This ticket is part of a larger issue to implement KPIs that measure certain aspects of the library.
An RFC regarding the larger issue can be found here.
This issue implements one of those KPIs.

The KPIs:

1. Turnover rate of the Library collection:
   1. number of new loans / number of loanable items
   2. number of renewals / number of loanable items

A detailed plan for the implementation can be found in the RFC.