Add multi-dimensional support about girth_mcmc

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Constraints on the discrimination parameters are backward with respect to the marginal methods. Need to update to maintain consistency between girth and girth_mcmc.

[eribean]

After some research, I'm going to add an initial guess method in girth that will seed the estimation. This will be a flagged option but should help maintain congruency between the two

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Relavant issue at GIRTH

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Added initial guess as an option that only affects multidimensional models.

options = {'initial_guess': True}

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Add the multidimensional grm model.

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This is finally closed with #13