[operators] remove MatrixBasedOperatorInterface and rename as_vector_array -> as_vector
[operators.numpy] add shortcut for NumpyMatrixBasedOperator.apply()
[operators] add error handling for apply_inverse()
[reductors] remove obsolete and unmaintained numpy_reduce_stationary_affine_linear
remove unnecessary calls to assemble()
[operators] add FixedParameterOperator
[parameters] small tweaks
[operators] improve implementation of assemble()
[operators] add documentation
[operators] improve documentation
[operators.numpy] remove unnecessary super() calls