This commit adds a new aggs-matrix-stats module. The module presents a new class of aggregations called Matrix Aggregations. Matrix aggregations work on multiple fields and produce a matrix as output. The first matrix aggregation provided by the module is matrix_stats aggregation. This aggregation computes the following statistics over a set of fields:
* Covariance
* Correlation
For completeness (and interpretation purposes) the following per-field statistics are also provided:
* sample count
* population mean
* population variance
* population skewness
* population kurtosis
Nicholas Knize