"boxplot" is not available as an aggregation

[hugorodgerbrown]

ES 7.x includes the "boxplot" aggregation, which will return the max, min and 25/50/75 percentiles (1st, 2nd, 3rd quartiles) values - the source for a boxplot chart.

elasticsearch_dsl does not currently support this, and raises an error if you try to create it:

elasticsearch_dsl.exceptions.UnknownDslObject: DSL class `boxplot` does not exist in agg.

[hugorodgerbrown]

Added in the missing Agg subclass in #1412