[ML] function_description set to mean for lat_long population detector

[peteharverson]

function_description is being set to mean in records for a lat_long detector with an over field. This should be set to lat_long, in line with the behaviour for non-population analyses:

Found in 7.0.0-alpha1, but I believe this has been the long-standing behaviour.

Detector config used:

"analysis_config": {
    "bucket_span": "15m",
    "detectors": [
      {
        "detector_description": "lat-long(location)",
        "function": "lat_long",
        "field_name": "lat-lon",
        "over_field_name": "apache2.access.remote_ip",
        "detector_index": 0
      }
    ],
    "influencers": [
      "apache2.access.remote_ip"
    ]
  }

Note that function_description is used in the UI as a display hint field, and affects for example the columns in the anomalies table.