divide by zero bug in elliptical_exact.pyx

[astrofrog]

I don't know when this got introduced, but I'm getting failed tests due to this function in elliptical_exact.pyx:

def circle_line(double x1, double y1, double x2, double y2):
    '''Intersection of a line defined by two points with a unit circle'''

    cdef double a, b, delta

    # Find the slope and intercept of the line
    a = (y2 - y1) / (x2 - x1)

When x1 == x2 (which happens for vertical lines of course) this give a divide by zero error.

Actually I'm also getting failed tests on master having to do with the changing API for encloses methods, but that will be resolved in my PR #11.

[astrofrog]

The errors were due to particular alignments of the pixel edges with the aperture. I've fixed this, and all the tests seem to pass. I'm going to add some more thorough tests after this.