LearningCurve visualization: handle case where skill names are a single character

[personalized-learning-2]

Because we generate the output image files using the skill names as the file prefix, we were violating the requirement to have a prefix of 3 or more characters (https://docs.oracle.com/javase/7/docs/api/java/io/File.html#createTempFile(java.lang.String,%20java.lang.String,%20java.io.File)

Line 816 of LearningCurveVisualization.java: File.createTempFile().

We append "_" but that just takes us to two characters in the single-char case; need to pad to ensure at least 3 characters.