fixes revstring parsing on clean tags
[algorithms.greedy] ensure to return the right reduced discretization
[demos] fix thermalblock gui
[la.gram_schmidt] fix check for linear dependence
[la] fix circular import
[discretizations] add with_projected_operators method
[la.interfaces] add missing pass
[reductors.linear] ensure that error_product is assembled
[reductors] make use of with_projected_operators
[operators.interface] allow to add int(0)

This is necessary to sum( ) a list of operators
[operators.basic] small tweak
[operators.constructions] make LinearLincombOperator actually applyable
[parameters.base] fix rename_parameter
[reductors.linear] allow to reduce DuneLinearEllipticCGDiscretizations
[playground] proof of concept for a dune backend
[parameters.base] add __len__() to ParameterType
fix typo
[la.gram_schmidt] fix for product != None
[discretizers.elliptic] use h1 semi-norm as h1_product
[core.defaults] decrease tolerance of bicgstab