Visualization: if column 'XValues' not provided and multiple conditions should be plotted automatically

If in the visualization table, the column XValues is not provided, it is inferred:

• if in measurement file measurement time is not constant, then XValues = 'time' ->plot time course
• if its constant, then dose response plots are plotted. For now, only the first column in the condition file is taken into account.
  Expand to automatically plot all conditions. Ideally like subplots B and C in:
  https://journals.plos.org/ploscompbiol/article/file?type=supplementary&id=info:doi/10.1371/journal.pcbi.1007147.s005

see conversation in PEtab-dev/PEtab#283.

Start with the visualization-table handling for the XValues column, then review how measurement time and the first condition-file column currently select plots. Use the PEtab-dev/PEtab#283 conversation and the linked supplementary example as context. Done means automatically plotting all conditions, including multiple dose-response subplots when XValues is absent.