Lambda constrains

Derek David found that while running with a prior

lambda_1 = Constraint(name='lambda_1', minimum=0, maximum=5000)
lambda_2 = Constraint(name='lambda_2', minimum=0, maximum=5000)
lambda_tilde = Uniform(name='lambda_tilde', minimum=0, maximum=5000)
delta_lambda_tilde = Uniform(name='delta_lambda_tilde', minimum=-5000,maximum=5000)

The posteriors include negative lambda_1 and lambda_2 values. Even with frequency-domain-source-model=lal_binary_neutron_star and the prior dict being set. This is clearly buggy behaviour and needs fixing.