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This allows (and changes the current examples) to exit with status 99 to
skip a test instead of outputting a special string ("NumPy missing").

This also fixes the eigen test, which currently fails when eigen
headers are available but NumPy is not, to skip instead of failing when
NumPy isn't available.

Previous version would give false 'OK' if, for example, we were supposed
to get [1, 2, 3] but instead got [2, 1, 3].

Fails --- next commit will tighten test.

Passing a non-contiguous one-dimensional numpy array gives incorrect
results, so three of these tests fail.  The only one passing is the
simple case where the numpy array is contiguous and we are building a
column-major vector.  Subsequent commit will fix the three failing
cases.