Add conflation metric

[slawler]

Per our discussion, let's start with the following metrics included in this working schema:

{
    "metadata": {
        "ripple1d_version": "v0.4.1",
        "source_network": {
            "nwm_hydrofabric_data": "nwm_flows_v3.parquet",
            "nwm_hydrofabric_version": "2.1"
        },
        "source_ras_model": {
            "item": "stac_key",
            "source_ras_model_item_id": "aaw0eoi2wfnf",
            "source_ras_files": {
                "geometry": ".g01",
                "forcing": ".f02",
                "project-file": ".prj"
            }
        },
        "conflation_units": "feet"
    },
    "reaches": {
        "1293180": {
            "us_xs": {
                "river": "Anthony Branch",
                "reach": "Reach-1",
                "min_elevation": 419.74,
                "max_elevation": 439.64,
                "xs_id": 21994
            },
            "ds_xs": {
                "river": "Anthony Branch",
                "reach": "Reach-1",
                "min_elevation": 348.98,
                "max_elevation": 384.89,
                "xs_id": 1685
            },
            "low_flow_cfs": 249,
            "high_flow_cfs": 774,
            "eclipsed_nwm_reaches": [
                "1293098"
            ],
            "overlap_nwm_reach": {
                "id": "1293099",
                "overlap": 0.000003
            },
            "metrics": {
                "nwm_reach_coverage": {
                    "start": 0.33,
                    "end": 0.7
                },
                "reach_lengths":{
                    "nwm": 12030,
                    "ras": 15008,
                    "nwm_to_ras_ratio":"12030/15008"
                },
                "thalwegs":
                {
                    "min":0,
                    "mean":10,
                    "max":100,
                    "percentiles":"tbd"
                }
            }
        }
    }
}