Welford algorithm extension

This MR extends the Welford implementation to support the third-order central moments of a (vector) field.

It also adds a basic test case for different setups of the Welford (different spatial dimensions and different orders of validation).

requested review from @holzer

assigned to @schottenhamml

added 1 commit

• f58babba - Fix flake warnings.

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merged

mentioned in commit c6fc87d8