Library Statistics - 8. Literature requests

[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

8. Literature requests:
   1. Average Efficiency: literature request acceptance waiting time / number of literature request accepted.
   2. (Is already possible with Invenio ILS, but no time series data available) Fulfillment: percentage of accepted vs. declined literature requests (differentiate decline reason 'available in catalogue' from other reasons).

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