gw/likelihood.py: prior type

Hi,

there may be a slight issue in testing prior type in gw/likelihood.py: the prior setter at https://git.ligo.org/Monash/tupak/blob/master/tupak/gw/likelihood.py#L110 makes prior an empty dict() if prior=None. This cause the check _check_prior_is_set(self) at https://git.ligo.org/Monash/tupak/blob/master/tupak/gw/likelihood.py#L102 to pass even when the likelihood class is initialised with prior=None.

In addition, it seems that this _check_prior_is_set(self) should not be needed as _setup_distance_marginalization() and _setup_phase_marginalization() can add a default prior in the event it's missing. And the calls to prior[<param>] = 0 at https://git.ligo.org/Monash/tupak/blob/master/tupak/gw/likelihood.py#L70 and https://git.ligo.org/Monash/tupak/blob/master/tupak/gw/likelihood.py#L76 should instead be self.prior[<param>] = 0

I need to add the disclaimer: my overwhelming prior belief is that I simply mis-read the code as while trying to understand it...