Setting priors

[j-faria]

My original goal for how to set the priors was

• there's a default set of priors, which we can use with `#include "default_priors.h"
• after doing that, the user could change a few priors individually, either inside or outside of the RVmodel constructor, depending on whether they depend on the data

But that has created a few complications and was the cause of #13 and #15 at least.
So I was thinking of refactoring things to be able to do

#include "default_extra_priors.h"  // sets Cprior, Jprior, maybe slope_prior
#include "default_GP_priors.h"  // sets the eta priors
#include "default_planet_priors.h"  // P, K, e, phi and w priors
#include "default_hyper_priors.h"  // sets log_muP_prior, wP_prior, log_muK_prior

Maybe the last include should also set P and K priors, but it will get confusing fast...
Let's use this issue to discuss

[eggplantbren]

Hmm. I think the best way to set all the options would be to have a type (class) with all the settings in it, and handle all the specific sets of settings when needed by looking up the contents of the settings object. That would be a lot of work, though.