Implement limit case of R2D2M2CP for P

[ferrine]

close #170 to enable strict coefficient sign in the model

with pm.Model(coords=dict(a=range(2))) as model:
    std, beta = pmx.distributions.R2D2M2CP(
        "beta1",
        1,
        [1, 1],
        dims="a",
        r2=0.8,
        r2_std=0.1,
        positive_probs=[0.5, 0],
        positive_probs_std=[0.3, 0],
        centered=False,
    )

[ricardoV94]

Failing test should be fixed by #183

[ferrine]

Just rebased

[ferrine]

I found an issue if all the positive_probs are at the limit case