[operators.solvers] add solve_linear and solve_linear_numpy_bicgstab
[operators] add sparse attribute to LinearOperatorInterface
[discretizations.linear] use solve_linear as default solver
[reductors.linear] allow discretizations without solver attribute
[discretizers.elliptic] add l2_product to discretization
[la.numpyvectorarray] fix two assertions
[algorithm.timestepping] add first version of implict_euler()
[gui.glumpy] add GlumpyPatchWidget
[gui.qt] add visualize_glumpy_patch()
add license headers
[playground.demos] add demo solving a high dimensional parabolic problem