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+New: The class SolverCG now supports the interleaving of vector operations
+with the matrix-vector product. The prerequisite is an associated `MatrixType`
+class to provide a `vmult` class with two `std::function` objects to specify
+the operation before and after the matrix-vector product, and a
+`PreconditionerType` class that provides a function `apply_to_subrange(const
+unsigned int, const unsigned int) const` that can selectively apply the
+precondition on a part of a vector. For optimal performance, the matrix and
+preconditioner types need to agree on suitable sizes for the sub-ranges.
+<br>
+(Dmytro Sashko, Martin Kronbichler, Peter Munch, 2022/05/09)