Update docstring
Fix way lambda function is defined
Add Conv2DSpace.default_axes, use it to avoid conversion errors

In particular for the case Conv2DSpace(b01c) -> VectorSpace ->
Conv2DSpace(c01b), because the VectorSpace has no way to know
what it is a flattened version of, and the second Conv2DSpace does not
now either.
Change tests to use Conv2DSpace.default_axes instead of ('b','c',0,1)

Also add test for indirect conv2d -> vector -> conv2d conversion
Merge pull request #382 from lamblin/fix_convert_fn

Make Conv2D to Vector conversions independent of axes