Invariants

[cscherrer]

This PR adds two invariants:

• logdensity_def(μ, x) ≈ logdensity_rel(μ, β, x) (in test_interface)
• logdensity_rel(pushfwd(f, μ), pushfwd(f, ν), x) == logdensity_rel(μ, ν, inverse(f)(x)) (in test_pushfwd)

Currently some tests are failing. I think these laws are correct, so we should examine the codebase more carefully and get this in line.