JENKINS-21493

The changes added to XMLSeries.loadSeries in 1.6 to handle nodesets where the label and value were in separate nodes in the nodeset interfered with the simple case supported previously where the labels came from the node name where nodes had numeric text content.  

This changeset ...
* Adds a test that demonstrates JENKINS-21493 before the refactoring
* restores support for that basic use case
* refactors the use of Double.parseDouble to use java.util.Scanner instead
* renames affected tests to more accurately, albeit verbosely, describe the use case being asserted
* and some whitespace cleanup for readability.