Make sure betaprim is initialised

This can be used to give a smart default for the beta_prime_input value so it is important that this is initialised.

Also avoids a potential bug whereby we can use an uninitialised value for gryfx. We do not currently hit this as we read in the box namelist even if not running in box mode. The patch here makes the code more defensive in case we ever choose not to read box namelist in non-box mode.