Problem 2083. Independent direction vectors of a plane

This is unfortunately suffering from many issues.

1. Many cases (planes parallel to one or two axes) aren't covered in the test suite.
2. Exact equality tests are used for floating-point comparisons, which can lead to spurious failures.
3. assert(dot(v1, v2) ~= 0) doesn't make sense, as remarked.
4. Neither would assert(dot(v1, v2) == 0), however, since the direction vectors are required to be (linearly) independent, not orthogonal.

Christian, I think the assertion dot(v1,v2)~=0 is placed to ensure that the two vectors are not parallel and therefore are independent.

I have modified the assert conditions and added some test cases to reflect planes parallel to axis/axes.

Thank you, Dyuman!