Use Measurement/DataSet name in plot_dataset and load_by_run_spec

After https://github.com/QCoDeS/Qcodes/pull/1741 (and partially https://github.com/QCoDeS/Qcodes/pull/1715) the fact that a `Measurement` has a `name` which after the measurement becomes also the `name` of the resulting `DataSet` should probably be captured in `plot_dataset` (get it's place on the plot) and `load_by_run_spec` so that users can search for a dataset by it's name as well.

Locate the plot_dataset and load_by_run_spec entry points, then read PRs 1741 and 1715 to understand how Measurement and DataSet names are currently propagated. Confirm the existing tests for plotting and run-spec loading, and consider the work done when a DataSet name is shown in the plot and can be used to find the dataset.

Written by the indexing model from the issue text.