Fix as_source_array/as_range_array for BlockRowOperator/BlockColumnOperator

[sdrave]

Fixes #624.

[sdrave]

@pmli, could you verify that the code is now working. (We don't have a test for this ATM ..)

[pmli]

It seems to be working

>>> from pymor.basic import *
>>> from pymor.operators.block import BlockRowOperator
>>> import numpy as np
>>> A = NumpyMatrixOperator(np.eye(2))
>>> B = BlockRowOperator([A, A])
>>> B.as_source_array()
<pymor.vectorarrays.block.BlockVectorArray object at 0x7fa1a195a9e8>
>>> B.as_range_array()
NumpyVectorArray([[1. 0.]
 [0. 1.]
 [1. 0.]
 [0. 1.]], NumpyVectorSpace(2))