This commit fixes several bugs in the the implementation of the Python interface of the MeshAccessor, that was introduced by commit 8178b1e40.

It was not possible to create a mesh accessor object, because there was no constructor nor function to create it.
Thus a constructor was added.
The second bug was in the wrapper for the resizeNodes() function, which actually redirects to the resizeConnectivity().

The last point, was not really an error.
Because of the implementation by functionpointers, the default arguments of the function was lost.
So This was changed.