[la.numpyvectorarray] minor docu update
[la.blockvectorarray] add first draft to playground

This does already work and seems to do the right thing (apart from the
methods that are not implemented, of course).
[la.blockvectorarray] fix __init__
[la.blockvectorarray] add length check
[la.blockvectorarray] simplify copy
[la.blockvectorarray] implement append()
[la.blockvectorarray] implement remove()
[la.blockvectorarray] implement almost_equal()
[la.blockvectorarray] implement scal()
[la.blockvectorarray] implement dot()
[la.blockvectorarray] update check in l2_norm()
[la.blockvectorarray] minor update of axpy()
[la.blockvectorarray] make BlockVectorArray importable from playground.la
[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