Changing more .view to .astype

More ``.view`` problems in ``aperture.py`` causing a bug in the masked photometry. 

(Size of ``x_mirror`` on line 696 gets twice the desired as the code asks for a ``.view(int32)`` of a float64, which  gives back two elements for each element in the original array.)

Should be merged before PR #77, as it depends on it.