Support StandardDeviation operator

[damieng]

Add a new ElasticMethod method for supporting the Standard Deviation operation in ElasticSearch facets as LINQ does not provide a method to do this.

query.GroupBy(g => 1).Select(y => ElasticMethods.StandardDeviation(y.Something))

Not recommended to add a top-level operation for this as there is no corresponding IEnumerable/IQueryable overload and the GroupBy pattern is more efficient when selecting multiple entries.

[damieng]

Provide this and others with the move to aggregates instead.