[parameters.base] allow None as entry of inherits in build_parameter_type
[algorithms.timestepping] allow rhs=None for explicit Euler algorithm
[discretizations.nonlinear] allow None as rhs
[discretizers.advection] allow problems with rhs==None
[analyticalproblems.burgers] set rhs to None
[discretizations.nonlinear] add mass matrix
[operators.constructions] allow None as operator in project_operator and rb_project_operator
[reductors.basic] fix for the case that operators contains None
[discretizations] move StationaryLinearDiscretization to basic module
[discretization] move InstationaryNonlinearDiscretization to basic
[discretizations] rename StationaryLinearDiscretization -> StationaryDiscretization
[discretizations] rename InstationaryNonlinearDiscretization -> InstationaryDiscretization
[discretizations] separate DiscretizationInterface from default implementations
[discretizations] add linear attribute to discretizations
[discretizations] improve logging
[algorithms.timestepping] add ImplicitEulerTimeStepper
[playground] update parabolic demo