Implement dual-loop method for getSummaryScoresForRange

Need to write tests, but believe this should be correct (maybe not efficient)
Add very basic test and test data for CombinedDataSource
Add additional test of CombinedDataSource
Have Segment inherit from BasicScore
Some scratch work on uneven boundaries
Believe we have a working model of CombinedDataSource.

Only supports addition and subtraction right now, should be easy enough to extend. Not hooked into UI.

IGV-1931