Add: Allow plotting by observableID & simCondID at the same time

Could in some cases be useful to plot e.g. a single observable for only a few conditions.
So far an error rises: "Plotting without visualization specification file and datasetId can be performed via grouping by simulation conditions OR observables, but not both. Stopping."

No files, tests, or entry points are named. Start by locating the plotting code that emits the quoted error and trace how observableID and simCondID grouping are handled; done means both filters can be used together without that error, with the resulting plot limited to the selected observable and conditions.