[algorithms.basisextension] let algorithms return a data dict.

The data dict contains a 'hierarchic' item, which is True if the
basis extension was hierarchic.
[reductors] add subbasis_reduction argument
[algorithms.greedy] provide subbasis_reduction if extension was hierarchic
[reductors.linear] fix typo
[reductors] rename argument subbasis_reduction -> extends
[algorithms.basisextension] bugfix
[algorithms.basisextension] fix pod_basis_extension
[reductors] return reduction_data as third argument
[demos] update to new reductor interface
[reductors.linear] reorganize residual computation to take advantage of old_data
Merge branch 'use_old_rd'