Save as... fails with untagged time points

Problem: Error in PlanarLarvae.Datasets:

MethodError: Cannot `convert` an object of type
  Vector{Union{Nothing, Int64}} to an object of type
  Union{Int64, Vector{Int64}}

Line number is not significant here, as this was spotted on the dev branch.

Steps to reproduce: Include an untagged (or partially tagged) larva as to be exported, and save with the "Save as..." button.

Expected behavior: Either exclude the untagged time points or properly handle the no-tag case.