function score: fix cast in Gaussian decay function

Also fix the test
FunctionScoreTests#simpleWeightedFunctionsTestWithRandomWeightsAndRandomCombineMode
which sometimes failed due to rounding issues. Make sure
only floats are returned as scores to assure ratio of
expected and returned score is 1.0f.
Aggregations: Adds ability to sort on multiple criteria

The terms aggregation can now support sorting on multiple criteria by replacing the sort object with an array or sort object whose order signifies the priority of the sort. The existing syntax for sorting on a single criteria also still works.

Contributes to #6917
Replaces #7588