Partial derivative tests

[sixpearls]

In the calculus tests, the partial derivatives didn't seem to work if one of the other dimensions was of degree 0. This is why the multidimensional splines in the fixture are of degree 1 or more:

ndsplines/tests/test_calculus.py

Lines 10 to 11 in c771982

	[_make_random_spline(2, [kx, ky]) for kx in range(1,4) for ky in range(1,4)] + \
	[_make_random_spline(3, [kx, ky, kz])

I am not sure why this would be the case, and may be worth investigating.