Changed LeafAggregation to MetricsAggregation

Metrics aggregations are now not restricted to numeric values.  The existing MetricsAggregation has been renamed to NumericMetricsAggregation. Only NumericMetricsAggregations can be used in the sort parameter for bucket aggregations
Updates to docs to explain difference between numeric and non-numeric metrics aggregations
More review comment chafes

Geo bounds aggregation now uses BigArrays so it can support multiple bucket ordinals.  Also now returns two GeoPoints (topLeft and bottomRight) instead of BoundingBox Object