Moving XMLCodec constants to their own classes

[sambish5]

#515 (comment)
Done from this comment left on 515. Moved AnswerFile constants and method constants to separate classes.

[jdcove2]

Some suggestions...

• "AsnwerXMLFileElementNames" -> "IbdoXmlElementNames"
• "IBDOMethodNames" -> "IbdoMethodNames"
• Javadoc all of the constants in the above classes.
• Make the above two classes final.
• Move ...ATTRIBUTE_NAME fields to IbdoXmlElementNames class.

[drivenflywheel]

Some suggestions...

• "AsnwerXMLFileElementNames" -> "IbdoXmlElementNames"
  Disagree on this one. IBDOs don't have element names, and these names are specific to our UnitTest/ExtractionTest/RegressionTest XML answer files.
• "IBDOMethodNames" -> "IbdoMethodNames"
  Meh, I find IBDOMethodNames to be more readable
• Javadoc all of the constants in the above classes.
  IMO, constants should only have Javadoc comments if those comments clarify usage or ambiguity.
• Make the above two classes final.
• Move ...ATTRIBUTE_NAME fields to IbdoXmlElementNames class.

[drivenflywheel]

Good first pass, but I think we can further improve the readability with minimal effort.