Sierra 2.4.0: started migration to Sierra 3.0

We started migration to Sierra 3.0, a new refactored version of Sierra program which includes brand new NGS (via codon frequency table) and HIV-2 support. Note the NGS and HIV-2 supports are only included in Sierra 3.0.0b1.

We cherry-picked a few changes from Sierra 3 to Sierra 2.4.0. These changes are necessary for our refactored frontend code be able to work with the Sierra 2 interface.

1. Attribute mutationPrevalences and algorithmComparison are now not only available in mutationsAnalysis node but also in sequenceAnalysis node.
2. Every attribute under viewer node is now also available under the root node.