fix invalid length error with meshio.CellBlock

In commit [60d11b70] of meshio the length of `CellBlock`, a subclass of
`namedtuple`, is redefined to the length of the `data` attribute. This breaks
`namedtuple._replace`, where the length of the newly created `CellBlock` is
verified. This patch circumvents the problem by using the `CellBlock`
constructor directly instead of `namedtuple._replace`.

[60d11b70]: nschloe/meshio@60d11b7

nutils/mesh.py

@@ -357,7 +357,7 @@ def parsegmsh(mshdata):
       if keep.all():
         renum = numpy.arange(len(cells.data))
       else:
-        msh.cells[icell] = cells._replace(data=cells.data[numpy.hstack([True, keep])])
+        msh.cells[icell] = type(cells)(type=cells.type, data=cells.data[numpy.hstack([True, keep])])
         renum = numpy.hstack([0, keep.cumsum()])
       renums.append(renum)
     for name, (itag, nd) in msh.field_data.items():