Add gmv aggregate

[texodus]

This PR adds a new aggregate gmv ("gross market value"), which for any node in a Perspective group_by / split_by tree:

• Leaf (full row depth): plain signed sum.
• Parent: Σ over immediate row children of |sum(child_subtree)|.

The parent uses each child's raw signed sum, not the child's already-rolled-up gmv value. This makes parent elements ≠ Σ|child_gmv|.

This specialized aggregation will be noticeably slower than regular sum due to recursively recalculating gstate for each depth, so use it wisely.

[nmichaud]

GMV = sum(abs(MV)). It's never negative. Rolling it up can be tricky depending on how you want to treat positions on the same names across multiple subgroups (ie net them or treat as separate).

[texodus]

@nmichaud This implementation nets by the last group_by column, for all ancestor groups/splits, which is why it is slow (it is re-grouping for every ancestor level). E.g., the net column in the test case is "symbol".

The last column's aggregate is "sum" instead of "sum abs", because this implementation is from a spec. You can create an expressions column using the abs() function to calculate this:

{
    "expressions": {
        "abs(MV)": "abs(\"MV\")"
    },
    "aggregates": {
        "abs(MV)": "gmv"
    }
}

... or y'all can change the spec (@timkpaine).

[nmichaud]

Ok that makes sense. I wouldn't call it GMV, at least without calling out that you need to use it on an abs(x) column.