Cross product primitive implemented

[parsa]

Part of issue #2

[hkaiser]

The NumPy docs say (see https://docs.scipy.org/doc/numpy-1.13.0/reference/generated/numpy.cross.html):

The cross product of a and b in R^3 is a vector perpendicular to both a and b. If a and b are arrays of vectors, the vectors are defined by the last axis of a and b by default, and these axes can have dimensions 2 or 3. Where the dimension of either a or b is 2, the third component of the input vector is assumed to be zero and the cross product calculated accordingly. In cases where both input vectors have dimension 2, the z-component of the cross product is returned.

IOW, we should add support for a cross product of 2 dimensional arrays as well.

[parsa]

After experimenting with NumPy a bit I found these differences with my current implementation that I have to address:

• Vectors must have exactly 2 or 3 elements (current code accepts vectors with fewer than 3 elements)
• It's possible to perform cross product on a vector and a matrix (Not possible currently)

np.cross([1, 2], [[4, 5], [7, 8], [1, 2]])

array([-3, -6,  0])

• The matrices can have 2 columns, too

np.cross([1, 2, 3], [[4, 5], [7, 8], [1, 2]])

array([[-15,  12,  -3],
       [-24,  21,  -6],
       [ -6,   3,   0]])

[hkaiser]

These three tests above don't look complete.

[hkaiser]

Very nice, thank you!