Add indication of lowest validation loss epochs on loss plots

This MR closes #70 (closed) by adding vertical line indicators for the epochs in which the validation loss is at its lowest. The vertical lines fade as we go from the lowest, to the second, third and (up-to) fourth lowest values.

tests/test_cli.py

@@ -478,7 +478,7 @@ def test_experiment(temporary_basedir):
         )
         == 1
     )
-    assert (output_folder / "model" / "trainlog.pdf").exists()
+    assert (output_folder / "trainlog.pdf").exists()
     assert (
         len(
             list(