Null identified larvae cannot be selected

Problem: Moving objects with id 0 cannot be selected from the assay plot.

MWT has 1-indexed moving objects and, to avoid union types that can accommodate undefined values (Nothing), LarvaTagger.jl encodes undefined larvae with a null id.

FIMTrack has 0-indexed moving objects and the introduction of support for FIMTrack files revealed the issue.

Proposal: Either admit a union type or pick a different unrealistic value to represent undefined larva id.

This issue was last-minute fixed in commit ec8bca02 prior to v0.3 release.