[algorithms.greedy] do not fail for empty samples
[algorithms.greedy] add output
[algorithms.greedy] do not return max_err or max_err_mu
Merge pull request #85 from ftalbrecht/let_greedy_work_for_empty_samples

[algorithms.greedy] do not fail for empty samples
[la.blockvectorarray] improve BlockVectorArray implementation.

Should now completely implement the interface.
[tests] fix valid_inds_of_different_length
[tests] add fixtures for BlockVectorArray
[operators.block] add first draft to playground
[operators.block] make BlockOperator importable from playground.operators
[reductors.block] add first draft of GenericBlockRBReconstructor
[reductors.block] make GenericBlockRBReconstructor importable
[algorithms.blockbasisextension] add first draft to playground

This adds

* trivial_block_basis_extension and
* gram_schmidt_block_basis_extension
[algorithms.blockbasisextension] make importable
[operators.block] fix {source,range}_type construction
[operators.block] add missing _{source,range}_dims

Those are needed in assemble, for instance.
[algorithms.blockbasisextension] warn instead of fail on wrong option
[algorithms.blockbasisextension] log failed extensions
[operators.block] add sanity check to apply_inverse, refs #80
[operators.block] handle empty dimensions correctly
[reductors.block] update doc of GenericBlockRBReconstructor