ENH: Remove duplicate code in BinaryBall and BinaryCross

The BinaryBallStructuringElement and BinaryCrossStructuringElement are
special cases of structuring elements that are also available in
FlatStructuringElement.  In fact, the code is duplicated almost
line-for-line in FlatStructuringElement.  To remove duplicate code and
enable easier future enhancements, this patch removes the duplicate
code in the BinaryBall and BinaryCross and instead instantiates the
appropriate FlatStructuringElement internally to do the computations.
Thus, there should be no difference in computational complexity or
behavior, and all prior tests still pass.

Note that, previously, the BinaryCross only enabled the implementation
of structuring elements all of whose radii are 1.  This default
behavior has been retained.  But the new implementation now also
enables the specification of other radii for the BinaryCross since the
implementation in FlatStructuringElement also enabled that.
The test itkBinaryDilateImageFilterTest2 has been updated to also test
this functionality using a cross radius of 2.

Note that itkBinaryDilateImageFilterTest2 is currently the ONLY test
of the BinaryCrossStructuringElement in ITK.

Change-Id: I757108ffe06d8b98f1e67b1a1c4729a0bbcb1509
Merge topic 'StructuringElementConsolidation'

1582827 ENH: Remove duplicate code in BinaryBall and BinaryCross