Complete testing of GP covariance kernel functions

[axch]

At the time the gpgradient branch was merged, the automated testing in test/conformance/sps/test_covariance.py was incomplete, in that it did not cover the bias, linear, and product kernels. That did not seem to be a sufficient problem for preventing a merge, but we should in principle fill those in.

[axch]

Assuming there are no underwater stones (which assumption may be false, as @riastradh-probcomp tends to be pretty thorough), finishing this seems like a good exercise for @Schaechtle , who is also the most immediate beneficiary of those kernels being tested and debugged.