Add a second baseline group and a migration to port existing result sets

[arteymix]

Add a secondBaselineGroup to our DEA result set model and use it when serializing VO instead of populating it on-the-fly.

TODO:

• migrate existing result sets (script is ready, but that will affect baseline filling in production, so we have to hold it off)
•  fill the second baseline group in LinearModelAnalyzer
• remove the logic for populating baselines in ExpressionAnalysisResultSetDaoImpl

[arteymix]

@ppavlidis maybe you can have a look at the populating interaction baseline part? This is a bit challenging because factors are held in an unsorted collection and the final picked order has to match what is used in the contrast results.