Support for multiple grouped / aggregated columns in pivoting

[mattjubb]

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Description

• Added support for multiple grouping / aggregation columns via the pivot method. kept the old method contract as is but now delegates to shared logic
• renamed column1/column2/column3 to groupedColumn/pivotColumn/aggregatedColumn to make it more
  obvious what the meaning of each argument is
• not entirely sure the approach of concatenating the pivot value with aggregated column name ie "2012.Score" is the best, open to suggestions here.

Note: first time contributing to a project on github so let me know if i've done something wrong

Testing

yes added tests for 3 use cases 1) both multiple grouping and aggregation columns 2) multiple grouping and single aggregation 3) single grouping and

[benmccann]

Codacy is complaining that you're doing Collectors.toList here when toList is already statically imported. I'm fine either way to want it as long as it's consistent