Aggregations: Adds GeoBounds Aggregation

The GeoBounds Aggregation is a new single bucket aggregation which outputs the coordinates of a bounding box containing all the points from all the documents passed to the aggregation as well as the doc count. Geobound Aggregation also use a wrap_logitude parameter which specifies whether the resulting bounding box is permitted to overlap the international date line.  This option defaults to true.

This aggregation introduces the idea of MetricsAggregation which do not return double values and cannot be used for sorting.  The existing MetricsAggregation has been renamed to NumericMetricsAggregation and is a subclass of MetricsAggregation.  MetricsAggregations do not store doc counts and do not support child aggregations.

Closes #5634