Prop 684: Create Stable Agnostic Volume Splitting Gauges

This proposal would create new gauges for groupings of pools that do not differentiate between which incentivized stable token is present in the pairing.

## Background
This proposal expands on the proposal to create volume-splitting incentives for all pairings that differ only by spread factor by creating stable token agnostic gauges.

This allows the market to decide which Stable token from those included in a gauge is preferable to establish liquidity by matching incentive share to actual usage.

Unlike normal gauges, which only have one destination pool, Volume Splitting Incentive gauges cover a group of pools. Incentives are allocated to this grouping rather than the previous method of allocating to single pool gauges.

The proportion of incentives that go to each pool in the grouping is recalculated every epoch according to the volume that occurred in the pools over the previous day. This allows pools that are generating the most volume per unit of liquidity to increase incentives day on day without waiting for a new incentive proposal to pass.

A Stable agnostic volume splitting gauge allows Osmosis incentives to adjust towards the pools using the most popular Stable token and away from those with less adoption. This will ensure that incentives go toward the most popular stable token for use without the protocol forcing dominance of any one of those included in the grouping.

## Proposed Groupings
Link to Table

Forum Thread:https://forum.osmosis.zone/t/create-and-incentivise-volume-splitting-incentive-gauges/635