Tests for ratio gradient

[christiaanjs]

I finished the tests for gradients and ratios and included the WNV dataset. They now all pass with a few changes.

• Parsing was fine
• libsbn no longer zeroes out a root-child-branch gradient so removed the fix from that
• libsbn doesn't add the root node bound in the forward transform so do that
• libsbn includes the gradient of the transform Jacobian which we should not when trying to use with autodiff - see https://github.com/phylovi/libsbn/issues/264, worked around with https://github.com/phylovi/libsbn/commit/abe67685a1feda513ba7658e16b7c41cd56c0785