add lambda/python function callbacks to dune.fem.function.gridFunction

• deprecate dune.fem.function.global|Function and add that functionality to dune.fem.function.gridFunction
• simplify some code for the interpolation in spaces
• use the dune.fem.plotPointData function for the dune.fem.function.gridFunction decorator instead of the `dune.common plotting

With this final change the whole construction of gridFunctions is done in one place. This simplifies code in other places that needed to check what type of argument was passed in, e.g., if something was an ufl expression or a discrete function or a Python function etc.