Merge branch 'update_changelog' into 'NIFTy_6'

Add new Ops to ChangeLog

See merge request !486

ChangeLog

@@ -16,6 +16,8 @@ In addition to the below changes, the following operators were introduced:
 * PartialConjugate: Conjugates parts of a multi-field
 * SliceOperator: Geometry preserving mask operator
 * SplitOperator: Splits a single field into a multi-field
+* MatrixProductOperator: Applies matrices (scipy.sparse, numpy) to fields
+* IntegrationOperator: Integrates over subspaces of fields
 
 FFT convention adjusted
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