Fix/cartesian segments intersection

 This PR fixes a bug in Cartesian segment-segment intersection strategy.

**Bug:** when exactly one segment degenerates to a point and is collinear with the other segment, the strategy always sets the intersection count to 1.

**Fix:** check if the degenerate segment lies within the non-degenerate one
before setting the intersection count; if the point is outside the non-degenerate
segment declare the two segments as disjoint

Some additional test cases have also been added in the distance unit tests. Some of them have triggered the bug mentioned above.

Test cases related to this bug will be added in the disjoint unit tests once PR #129 is merged.