detector_focal_point does not always give its unit vectors

[kaizu]

The following part assumes that the detector_focal_point, p0, gives its units, y0 and z0:

https://github.com/ecell/scopyon/blob/master/scopyon/epifm.py#L504-L505

However, it's not true when the detector_focal_point is an origin, (0, 0, 0).