[operators] intial work on improved operator interface
[operators.interfaces] add global_names parameter to lincomb()
[operators.basic] bugfixes
[operators.interfaces] add apply_inverse method to OperatorInterface
[operators.numpy] update numpy operators
[operators] use apply_inverse everywhere
[operators] rename default implementation classes
[operators] improve lincomb() more flexible in case of coefficients=None
[playground] update dune demo
[operators] small tweak
[operators.interfaces] add documentation
[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