[discretefunction] add tuple discrete function
This commit introduces a new discrete function implementation which stores internally a tuple of given discrete functions. For this new discrete function a TupleDofVector is added which stores references to the sub DoF vectors.
Access to the sub discrete functions: df.template subDiscreteFunction< i >() Access to the sub dof vector: dofVector.template subDofVector< i >()
Note: TupleDiscreteFunctionSpace is hard wired as the underlaying discrete function space.
Note: The access to DoFs via dofVector[ block ][ i ] is slowly.