Added a `Pop` method to `dip::VarianceAccumulator`, and a `Reset` method to all accumulators.
Added a `Pop` method to `dip::VarianceAccumulator`, and a `Reset` method to all accumulators.

DIPlib/diplib,DIPlib/diplib,DIPlib/diplib,DIPlib/diplib,DIPlib/diplib,DIPlib/diplib