add python bindings for writer

At the moment we have a function in dune.grid

def writeVTK(grid, outputBase,
             celldata=None, pointdata=None,
             cellvector=None, pointvector=None,
             number=None, subsampling=None,outputType=OutputType.appendedbase64,
             write=True, nonconforming=False):

this will either write the file (write=True) or return a vtkWriter(name) object with a write method. The method can be called as a method on the grid (e.g. grid.vtkWriter(...)) or as a standalone function vtkWriter(grid,...).

There is also a sequenced writer:

def sequencedVTK(grid, name, celldata=None, pointdata=None, cellvector=None, pointvector=None,
                 number=0, subsampling=None, outputType=OutputType.appendedbase64):

which allows to write vtk() to generate a sequence of output files.

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Add a function

def vtkWriter(grid, outputbase,
             celldata=None, pointdata=None,
             cellvector=None, pointvector=None,
             writer="unstructured", collector="dafault", outputType=OutputType.binary):

to create an instance of the class:

template <class GridView, class DataCollector = DefaultDataCollector<GridView>>
class Vtk[Type]Writer

can then add special method for typical cases for writerType, Of course we can't write grid.vtkWriteranymore but that is perhaps not so problematic. This would return an object with a__call__` option. So one would have to write

from dune.vtk import vtkWriter
vtk = vtkWriter(grid,"result",pointdata=[...])
vtk()

Depending on writer this would generate a single file (and calling vtk() would always overwrite this file) or a sequence of files.

Any other suggestions?