Clean up ScaleWeightGroup class

[kdlong]

The different variations are stored in a vector in a predetermined order, but that order isn't advertised anywhere, and is accessed with unexplained "magic numbers." There should either be an enum explaining the indices, or the vector should be an unordered_map. There should also be a check in the getXIndex function that the entry exists.

Talking about these lines: https://github.com/kdlong/cmssw/blob/ImprovedParsing/SimDataFormats/GeneratorProducts/interface/ScaleWeightGroupInfo.h#L46-L64

There are also some unexplained "magic numbers" here: https://github.com/kdlong/cmssw/blob/ImprovedParsing/SimDataFormats/GeneratorProducts/src/ScaleWeightGroupInfo.cc#L42

Another small point is that the official CMSSW naming convention recommends not using "get" in the name of getting funcitons. So getDynNames, getScaleIndex --> dynNames, scaleIndex.

https://cms-sw.github.io/cms_coding_rules.html#2----naming-rules-1

@dteague are you able to implement this?