Fix compilation error after Jackson update

Original code stopped compiling when I updated the VIPSCommon dependency to version 2022.1. The compilation error is probably due to changes in jackson, a dependency which was bumped from from 2.9.8 to 2.13.1 in VIPSCommon in early 2022. Changing the type of the pestObservations variable was the easiest way to fix the compilation error. The tests run green, but there is probably a more correct way to do this.