Default bilby priors have no validate_prior method

In the input module, bilby_pipe calls the method validate_prior here. However, this is not a base method of the bilby.core.prior.PriorDict, but only defined in the CBC specific priors here.

This isn't a significant issue, given bilby_pipe is ostensibly for CBC analyses, but could cause some headaches for people downstream (though it can be switched off using the enforce-signal-duration, but that won't be obvious.

We could either

1. Add a more informative error message
2. Check the prior class before calling validate_prior
3. Fix the issue in bilby itself by adding a validate_prior empty method

What do people prefer?