Additional meta operator template: MultiAggregate

This task aims to add further codegen templates besides the existing SpoofCellwise, SpoofRowAggregate, and SpoofOuterProduct.

Spoof MultiAggregate: There are quite a number of scenarios where we have expressions like (a) sum(Y == check_min), sum(Y == check_max), or (b) overlapping expressions with large common subexpression, i.e., fusable operations over a shared input with multiple aggregation roots. Since SystemML's compiler/runtime only supports single-output operations (with very few exceptions), it would be beneficial to generate a single fused operator with a combined aggregate output, which is subsequently split via traditional right indexing operations.